My friend gets on my computer and changes my stuff the first time: I tell him not to do it ever again or he's getting lit on fire...
My friend changes my stuff on my computer again: I get a spray bottle and fill it with lighter fluid and a lighter to for a make-shift flame thrower... then I "throw" flames at the back of his head...
Either that or I would shoot him in the feet with an airsoft gun while he's sleeping.... both are acceptable forms of punishment for being a douche bag in my circle of friends.