They don't have to be "shit NOOBS" even if they want xp :dumb:
You can kill when suiciding too. I use choco's custom eboot which gives me normal points when killing and capturing flags, so I can kill without any risks of being banned at all. Also, I couldn't care less about being banned as mw3 is a shit game and it's only an account ban :p
And nope, killing with big xp gets you banned as you will get high on the leaderboards when killing. Suiciding does not kick you out as long as it's not more than 65k