I bet the search function would have throw a few topics about it...anyway.
Create a trigger, size to fit your desired area, anybody/present/repeatedly
In the on activation line write:
{_x setdammage 1} foreach thislist
You can also use this to make safe zones in CTF style maps by setting Blufor/Opfor present, so enemy players can't enter respawn zone.
Myke out
*typo sorted* - Planck