Can I throw away my ex-girlfriend's stuff if she says she doesn't want it?

There is statutory procedure that must be followed before disposing of abandoned personal property. Basically, you have to give her thirty days written notice that she has to pick up her property. At the end of the thirty days, you can sell the items or donate them to a charity. Before doing anything, I would recommend meeting with an attorney to go over the statute thoroughly so you do not get into trouble down the road.