If for some crazy reason i was using a urinal, i'd tun and piss on them, while giving them my boot to the kneecap, and drawing my knife and slicing them to ribbons. Oh yea, make sure to cover your parts with your off hand when you draw your knife.
In a stall (i ALWAYS use a stall, because i don't like whipping out my... gun in the middle of a room. and the backsplash issue.), if the door opened inward, i'd just push back and shove them away. then i'd decide from there. if the door opened outward, i'd probably piss on them and again, decide from there.
~tmm