The conveyor's scroll speed is stored in an entity's render color, so you could cheat and use env_render to change the color to 0 0 0 to make it stop.
X is whether to change scroll direction.
The speed value is broken up into the Y and Z values, Z contains 0-255, Y contains 256-32768.
I've never tried this myself though, and it won't affect the conveyor's actual speed. You'll need to use some tricks to prevent it from still pushing things, like a func_door that moves up to cover it, or a func_wall_toggle, if you can be sure that nothing will be touching it when toggled.
Note that triggering a func_conveyor will reset its scroll speed settings, so always apply that after triggering it.