Make a setting in JCS that makes all of the objects which are pushable in single player pushable in multiplayer as well (big box, big rock, pickup crates, trigger crate, barell, ammo boxes, powerup TV's, morphs, and everything else I haven't mentioned). These items are not pushable in multiplayer, so the player just passes through them, making the box and rock almost useless.
The setting would be optional, of course, like the 'Hide level in Home cooked levels list' setting.