Advice on my bestfriend's gf.

Heh.

Tell her that she should try asking her boyfriend or something, since you are busy with our own issues, such as homework, work, etc. Make up excuses, but be polite about it. Give hints that you're 'busy' or don't feel like talking to her. If she's "dumb", then just say it out straight-forward. I know some people have problems saying things out. But I don't.

Actually, if I was being annoying, one of my friends would tell me something along the lines of "You're being fucking annoying today, shut up.", etc. Point being, sometimes, even honesty is the best resolution, despite it being 'crude'. So just lie and give her hints and say crap like "Ah, I see. However, I'm rather busy today" [yada yada yada], I do that all the time, and whoever I don't feel like associating with, ends up leaving me alone.

I know this one girl that has a crush on me, and expects me to visit her every week on a Thursday, and I told her I can't because I'm always working, or busy with school work. It works. Just be polite about it.