How do I get out of this trap?

So I have this bff called Lisa. She's seem's all nice and sweet but really she's just a little *****. She has a good mate called Sonya. Sonya is nice but she would stick up for Lisa , any time , even if she know's Lisa is wrong. So any way Lisa pretty much uses me. She asks me for loads of stuff like clothes and food. She all-way's want's to sleep over my house. But if I say no to her demands , she fall's out with me and makes up some crap about me and tell's Sonya. And of course Sonya will stick up for Lisa and has a go at me , im worried she may hit me. So now I never say no to what Lisa want's because Im scared of her. But what makes it worse is that , Even if I do give Lisa what she want's she some times fall's out with me any way and tell's Sonya crap about me , stuff like '' summer (witch is me) just called me a *****''. And Sonya bi-leaves Lisa. And there is the fact that when Lisa fall's out with me she all-way's , Becomes mates with me again. And im to afraid to tell her I don't want to be her mate , Because she will Make up loads of crap about me and tell every one and Lisa is very popular so she could make every one turn against me if she want's to. So I feel trapped :'( im a complete wuss bye the way so I can't stick up for my self :( help me plz xxxxxx