I'd say Iceycool's case is borderline: relogging with a different character to get vengence might be nearly indistiguishable from randoming. I'd advise against it if you don't want to get admonished/stoned/banned.
But, that aside, the rule is: if someone attacks you randomly, you can kill them ONCE. If they random you agian, you can kill them again. BUT, you can't kill anyone for attacking someone else, and if you kill them more times than they random you, you;re randoming.
-Throbblefoot