You get used to it.

I had a level 70 following my Shaman(x5) around Ironforge yesterday spamming them with /Kiss. I said "I'd hate to lose your level 70 on account of harassment." And he stopped. A few minutes later I received an apology in a whisper LOL

However, if people are harassing you, you can report them.

Just tell them if they don't stop, you will report THEM... x5.