How to turn off zombie respawn either completely or just set it too really long time?

[NeXuS]

Like many others already mentioned, there is not much joy in clearing out areas if they'll just spawn right back, and this doesn't seem to be that type of game like Left4Dead or something where you just grab a weapon and start to maul down zombies and that's all there is to it.
But many people said they tried turning it off on their own server/in their own game but it didn't work they still happened to be back at the same spot.
Any suggestions? If someone could give me a link to a detailed description about the zombie settings that'd suffice too.

[mauriciows]

don't we need some better explained sandbox settings? These questions arise too often..