Yeah, I've got that problem except switch the genders (yes, that means I am a girl unless you already guessed from my username). I keep telling the guy no but he keeps going back later on. Maybe if I was mean to him, he'd take a hike.........
You can avoid her and start hanging around other girls if you can. Although it may also make her jealous and come at you more. Unless you pretend you have a girlfriend, then she'll either back off or try to tear the two of you apart. Some girls are so annoying. (I'm a tomboy so I don't hang out with girls that much anyway.)
Some girls are pitful and desperate and she may see you as the easiest person to use for a good grade. Normally, I hate being mean to people, but sometimes you just have to if you want to retain your sanity. Tell her to get lost, go find another smart person to bother. If that doesn't work, then try some combacks like "I don't tutor <insert insult here> people." When I insult, I like to use big words they are not likely to understand such as vapid, callous, heinous, loathsome, repugnant, revolting, vile, nefarious, declasse, vacuous, and others. And if she actually knows what you just called her, they yell at her, "Then why the heck do you need me to tutor you for?!?!"
Another idea will work for some girls. If the girl bugging you says words like "like" and "you know" WAY too much, then try mimicing her when saying no. An example is: Girl: "Yo! Mike! I like need a you know, tutor for math class. LIke, math is SOOOOOO hard!! ! Could you like help me and you can you know come over and all?" You: "Yeah, like, I don't like tutor annoying girls like cuz like its SOOOO annoying and a waste of you know, my time." Or you can just say "no" and if she persists, then mimic her speech.
If worst comes to worst, then either refer her to someone else who is smart, tell her to get lost, tell her to have a teacher tutor her, or even better, start charging her impossibly high prices to tutor her.
Best of luck to anyone who has to deal with annoying people like that. Grrrrr. Just thinking of that one guy is enough to make me angry. Oh well, I don't see him ever so haha to him. Anyway, hope this helps or at least gives you guys some ideas. -Power Girl