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In many fictional franchises we see people like the Flash who can move at speeds defying physics.

In my story I want to include a speedster, however I feel that they would be too overpowered for the setting.

So my question is how do non-speedster characters defeat those with speeds equivalent to those of say the Flash?

Because in many settings these characters are shown to be nigh invincible because no one other than speedsters can stop speedsters.

Don't use the Flash.

You are right speedsters like Flash are too OP. But it is cool to have people who are really quick. It might be even cooler to have them be just past the edge of what is humanly possible. My suggestions

1: Think of things you want your Speedster to do. For example, a person who could process fast and move fast might be just about impossible for a normal person to catch / hit. I know because I lived with a guy like that. After he ate your food he would head off across campus, giggling, staying just 10 feet in front of you, and sometimes even lying on the ground. But he could always scramble up and get away before you caught him. You would need a team to do it and by the time you assembled them he would be gone.

2: Rule out impossible QuickSilver type stuff.

3: Figure out weaknesses. For example someone who thinks and acts really fast might not take the most prudent course of action. Like failing to have a line of escape set up when he set about eating all the BooBerry cereal.

4: Defeat the speedster. Normal speed person can defeat the speedster by exploiting those weaknesses with a trap or the like, or an endurance contest. Or failing that having a team to surround him. Or coopt the Speedster to your side (BooBerry might work).

We can add some real science back into this to help handwave some things. Science and handwaving, what fun!

This speedster is fast, real fast, but where do they get the calories to burn for this speed? The faster they go, the more calories they need. Let's make a ballpark figure and say at full near light speed, they are burning 1000 calories a second. At this rate, an opponent merely needs to wait for the speedster to starve themselves and fall asleep or die. This means the speedster can only operate in short prepared bursts and must stop frequently for food and water. They can't just run around the world, they would burn up all their muscles before they got half way over the ocean.

And there you go, a plausible limitation that lends itself nicely into multiple possible story puzzles for heroes and villains to solve.

Every good hero has a fatal flaw. Even Superman has kryptonite, and is also kindof a coward.

For your speedster, maybe he's not very good at forward-thinking. The non-speedster only has to set up some bait and swing his bat -- the speedster hits the bat every time, and at those speeds, the blow does a lot of damage.

For that matter, your speedster can't afford to take a hit while he's moving fast at all. Even if a bug gets in his way, it'll be like a paintball or a bullet, depending on his speed. The non-speedster just has to throw some sand in the air, and it'll disintegrate the speedster as he moves through it.

Tie their shoe laces together?

In all seriousness, your best bet is to use their speed against them. It doesn’t matter how fast they’re running, if they fall over they won’t be running for much longer.

By far the simplest way of stopping them is to dig a 1ft by 1ft by 1ft hole, essentially a rabbit hole. If you’ve ever stepped in one, you’d know how easy it would be to twist or break your ankle. Now, imagine you’re running at these high speeds, not a care in the world, and step into one of these holes. At best, you’ve twisted your ankle and torn the tissues in your leg, you might instead break your ankle, your leg, damage the nerves, tendons, muscles, perhaps even the artery in your leg. A potentially fatal but easy to construct trap.

Going slightly bigger and more advanced, you could have the good ol’ pit fall trap, a large pit covered by a tarp and hidden using sticks and dirt. The speedster runs over that, they’re going too fast to stop and falls in the hole. The fall may not be lethal, they may just break their legs in a cruel twist of fate, but you can easily make it lethal by putting wooden spikes at the bottom.

Further options are trip wires, caltrops, raising low-barriers or wet/slippery surfaces. All of these would cause major issues for a speedster. I also remember a story, i don't know its true though, where during the second world war, piano wire was tied to trees or lampposts so that the wire stretched across roads. This wire was tied so that, if you were sitting down, it would be at neck height. The idea of it was to kill Nazi motorcyclists by decapitating them as they drove past, using the speed of their bikes against them. Because of how thin piano wire is, you wouldn't be able to see it and, by the time you did, it would be too late to stop. As i say, i dont know if this was true, its just something i’ve heard.

Taking inspiration from that idea though, you could have walls of clear glass, like French sliding doors. If a speedster ran through it at high speeds, they might be cut to ribbons by the shards of glass and have glass in their eyes.

If you were talking about in combat though, i suggest pike formations. Speedsters, like horses, aren't stupid enough to run into this big, sharp pointy stick. However, if you had these hidden in long grass, for example, and lifted them up when they got close, they may not be able to turn back in time and be skewered.

Have the speedster depend on some rare substance that is expensive in order for their power to work.

For example, speeding up may require quicksilver, which is another name for mercury. The speedster might be able to break a thermometer and drink the mercury inside without suffering any harm, but that will only last them five deconds of real time.

If you really want to get serious, the main resource that may be consumed by speeding is lifetime. In George R. R. Martin's Wild Cards, there is a speedster who uses his powers for OP speed akin to the Flash's. After what seems like just a few minutes to him, though, he has biologically aged a handful of years. A little more usage of that power and he would drop dead of old age.