1) Get a better vocabulary. "OMG UR FAT & UGLY!" is sooooooo 1997.
2) If you continually harass someone and they hurt themselves over it, yes, you can be sued. Whether or not they'll prevail is anyone's guess, but given the Lori Drew case, you could find yourself in hot water up to your eyebrows.
Bottom line: if you're worried you're going to be sued for doing something, you probably shouldn't be doing it.

