Someone is flirting with my girlfriend on face book, what should I do?

Okay so this kid a long time ago was friends with my girlfriend, i allow her to talk to certain guys, but then he got mean towards her so she was not friends with him anymore, so now hes like "random fact, i miss you:)" he wrote that on her face book, should i kick his ******* *** all over the street? should i talk to my gf about it? i have been doing kickboxing for 11 years and i dont know how to fight him with out hurting him, and i can only see what he types to her not what she types back, im also very good at hacking.. should i hack her fb account? or should i just not get bothered by it, shes very loyal btw and would never cheat on me.