Teleport players back to the center of the region when they leave
Solution 1:
You could set up a repeating command block with:
/tp @a[x=<X>,y=<Y>,z=<Z>,rm=<R>] <X> <Y> <Z>
Replace <X>, <Y>, and <Z> with the corrosponding coordinates of spawn, and <R> with the distance people can get from spawn before they are teleported back.