I'm going to cut off a friend over how she spent her money
I'd like to know why you choose to do that.
I helped her out and then she bought a new computer with mine and other people's money.
It is reasonable she should have used the money to spend on needed things. You should not be expected to fund other peoples lifestyles.
She said she'll pay me back, and that it's not a big deal to her
I can understand why you are still upset with her, especially since she doesn't seem to recognize that what she has done was wrong.  However, you should give her a chance to pay back the money, instead of just cutting her off.  She might have really needed a computer for other things, and might still plan to fulfill her promise.