The easiest way is to make a non-entry-able zone for enemies.
The intelligent way is: to let the enemies not have thought about to option to come close to the spawn zone, because
of having no advantage, being there.
e.g: i like using respawn scripts, where you respawn with the
same weapons, you were carrying before you got killed.
This makes you dangerous as soon as yourespawn.
Let soldiers respawn, where they havin good cover
Place objectives and respawn zones that way, that enemies
will not have any advantage on approaching the oponents
base, and even will get disadvantage because of that.
That's the real difference between a good and a bad mission
anyway
:edit - btw - i forgot to mention:
implement: no firing/kill zones there
That means: if you are somewhere, where you shouldn't be,
and you fire your weapon, you'll get a penalty or so - same
with killing others, where you shouldn't
~S~ CD