I have from time to time reacted in the same way, I've also at times taken the extremely forgiving option and just ignored them and moved on. But if someone continues to tag / steal my mobs I'll either report them for harrasment (which usually results in nothing but a warning and not much else) or take the 'like begets like' option and in turn tag their mobs. Sometimes this can result in them pulling too many mobs and dying. Its at that point I've succeeded, /kiss /lol /bye

If they abuse / swear at me aftrer I engage in polite conversation then even better for the GM report.