Getting rid of the headcrab!

I have replaced both the zombie and the fast zombie with my own characters. They work great, but there is one remaining issue: After they die, that damned headcrab still drops out!

I have made the crab invisible and made it not move, but the movement box still remains, so it blocks the player.

Anybody know how to get rid of this?

Or does anybody know a way to just make the headcrab NOT drop?