What should I do when a girl keeps hitting me for no reason?

I am a college student. Usually I hang around in the gaming center to play games with my friends.Then this girl who is taller than me just randomly hits me in the side. So much that this was getting to be a daily thing when she would appear there. This happened way too often so she was just doing it because I would not do anything back. She even once karate chopped the back of my neck that really hurt. and I asked her why she did that and she pretended that she had no idea what I was talking about. She has a boyfriend so its not like she likes me. after a couple more times. I told her off. So she stopped doing it. Than today she started doing it again. by hitting me with a water bottle to my face. asking me if I wanted water. I wanted to tell this ***** to stop as I was trying to play my game. So I told her ya I want the water. so she pulled my head back and told me she would water fall it so I just kept telling her no. she then lost interest and started hitting my friend who seems not to mind her hitting us for no reasons. But even though we are guys, we don't just hit each other all the time. What got me to want to tell her off is she almost tried to karate chop me in front of my neck, but she didn't even reach it, she did this without looking. I didn't want to be some pussy and walk away just because a girl was hitting me. So I took it like a man. But now it is getting annoying. Kind of confused if I should tell her off since it worked the first time or just walk away.