You could use an elevator with it?s func_tracks (wasn?t it?, help!!!) below any monster and set the monster flag to "Prisioner" so it won?t move...
The elevator could be a platform with a totally transparent ("{blue", I think) texture applied (and with the render mode set to texture and 255) so it wont be visible.
If you make the elevator 1 or 2 units high it won?t block the player if he tries to pass below the monsters and if you make it from a 32*32 (I think it?s the size of Headcrabs) it won?t make the player jump if he passes near the monsters.
The monsters would keep on jumping if they are killed but if you trigger the elevator moving to the floor (and standing there) whenever the monster is killed it won?t give any problems.
There is also an entity that pushes things, Trigger_push, I think. I never used it, but you could check it out in the entity guide.
Sorry for my english, I?m sleepy...