create a brush - tie it to entity - "trigger hurt" and set the damage inflicted to a HUGE! value - so it doesnt matter what their health or armour situation is...if they are falling through it, make it a big brush or a huge number will also work..
just thought I'd ask another question right here... how can I have the players spawn without weapons except a knive? And how do I place weapons you can pick up on the floor?