BMX freewheels and drivers?

What you are seeking is called a fixed gear. Some people just call it a “fix”

With a fixed gear there is no freewheel, whenever the rear wheel turns the pedals also turn.

Track bikes have fixed gears and no brakes. To slow down track riders have to resist the movement of their pedals. It is not possible to stop a track bike as quickly as a bicycle with caliper brakes but since nobody on the track can stop quickly this isn’t a problem; in fact brakes are not allowed in most track events.

Anyway, true track bikes have a fixed gear cog that screws directly on the hub then a plateau where a smaller diameter left-hand threaded lock ring threads on. This is so the cog won’t unscrew when the cyclist uses their legs to slow down. For strong track cyclists this is a necessity. Many road cyclists ride a fixed gear in the off season where they simply screw a track cog onto their road bike rear freewheel hub. There is no lockring to hold the cog on but since they have caliper brakes as well this is considered safe; road cyclists usually are not strong enough at backpedaling to unscrew the cog anyway. The lockring of English threaded bottom brackets has the same threads as English threaded freewheels so you can screw one of these lockrings on to help hold your track cog on. This right-hand threaded bottom bracket lockring is not as good as a left-hand threaded track lockring but it is better than nothing.

I am not that knowledgeable about BMX and freestyle riding but I’m not aware of the use of fixed gears on these bikes. It would make doing a fakie simple but it would make many other tricks impossible.

If you really want to try this order a track cog and find an English threaded (the most common kind) of bottom bracket lockring. Remove your freewheel, screw on the track cog and screw the lockring on top. Ride a bit to tighten the freewheel then tighten the lockring with a bottom bracket wrench.