There is a speed cap(obviously, durr), and an even more fair suggestion without the download of an additional file that would probably be recognized as a virus is to do the following:
.mod speed(console command for alterating speed of a player or of the GM itself) or using a third party program for hacking use the same rule - They set the speed, but if the speed changes, then the data is sent to the server which then changes the speed to the correct value. My idea is that they add an aura active on BGs and arenas that applies the knockback effect but it doesnt move the player(enough only for the server to detect, must be totally unoticeable by the player), and it also slows/accelerates his speed by a really small amount for a small period of time(1 sec?) and then restores its previous speed, thus if you're hacking with speed/fly you'll have your speed restored to the normal and the knockback will disable your fly.
To prevent that people on mounts get this knockback, fall and die, then just make it possible on BGs and arenas, places where you can't fly.
A good aura for this would be the old typhoon spell's effect.
Or we could just have an anti-hack or at least block this one *popular* hacking program that you can find in one google search.