I suppose it's too late for
you to act real cool and poker-faced (psychotic teacher, this guy?). If that doesn't work you could perhaps mention that people who say things like that in western countries generally "get their heads kicked in" if it's a stranger they were swearing at...and even friends would soon tire of that.
Personally, I don't know if I'd've been able to suppress a smile at their behaviour intially...but I would be showing signs that I was finding it wearing, the joke stale by now. Can I ask, were you shocked (visibly, to them), and did you overreact at first? They could be playing on that still, if you had.
Hmm as for the graffiti, make sure there aren't any board markers lying around (prise them from their lifeless fingers if necessary if you forgot to clear any extra markers away, and keep the ONE marker in your hand - better, trouser pocket - at all times. Hopefully they won't start shoving their hands into your trousers, but hey, that could be fun maybe!

).
As for the punching, are you black and blue or are they just playful taps? I've found footsweeps (ashi barai) soon cure students of the temptation to run at me (as does being pinned to the floor by my dirty shoes, or bodily thrown/dumped back into their seats I know that sounds violent but usually it is more "playful parent" kind of stuff, to begin with at least). I'm not saying beat the little darlings up severely or leave marks, or cause an accident where they bash their head open and sawdust spills onto a sharp desk edge, just show them who's boss and that you are not a defenceless dweeb. I doubt if even your jolly laidback principal expects you to stand there all lesson with "Punchbag" tatooed on whatever part of your anatomy. (BTW I'm presuming you're male, I'd be surprised if boy students took to hitting female teachers quite so freely). Above all, use force but not excessive force or anger - proportional response, calm and collected.
Sorry that I can't give you many more pointers, but as long as the whole class is misbehaved at least you don't have the sort of problem where e.g. two girls who really don't want to be there and would rather die than smile let alone study English start complaining (the cheek!) about not "being able to study" when Damien 666 starts playing up (yes, he was the little sod who'd lay into me every class).