If you use ULX I think there is a plugin you can get to do it.
The code that Anonymous just posted is for Gmod Evolve.
Yes, there is a ULX plugin to do it: http://www.zombiemaster.org/smf/index.php?topic=10138.0 But it's for an old version of ULX (1.x), and won't run, even with ULib's compatibility layer.
Does anybody know if there is an updated script like that for new ULX? I would appreciate it.