Originally posted by sarteck:

Your best bet is to build defensively. Set up "fences," dig trenches, plop down a few spikewalls,

If you are not opposed to using cheats, you can use the console and enter "devcommands" then either "killall" to kill all nearby enemies or "stopevent" to make the raid stop.

[EDIT] But there is no setting to lower the frequency of raids.