Change the player's jump height
You can use the player member jumping_counter to limit the player's jump to less Olympic heights.
The jumping_counter increments from 0 (not jumping) to 1441 as follows:
0 181 361 541 721 901 1081 1261 1441
By manually setting the counter to 0 when the counter reaches one of these values, you can decrease the duration of the jump by effectively cancelling it mid-jump.
You'll need a pair of gamevars: one to hold the value of jumping_counter and one to contain the value 0.
gamevar temp 0 2 gamevar ZERO 0 0
Inside the APLAYER actor, use the following code to cut the jump duration in half:
getplayer[THISACTOR].jumping_counter temp ifvare temp 721 { setplayer[THISACTOR].jumping_counter ZERO }