20.04.2012, 23:36
Use MapAndreas to check if they're off the ground, GetPlayerVelocity to check if they're going upwards and a timer so that if this continues for 2 seconds or more (maybe 4 seconds or more, you want to avoid false positives), then kick/ban them (or alert online admins). Or if you're feeling more merciful, instead of kicking and banning them right away, warn them and use SetPlayerPosFindZ to put them on the ground.