At 7 years old you can't break it, she has to want to change it. If she doesn't want to quit, you can still do things to make it more difficult and undesirable for her to suck her thumb.... but the behavior won't change. She'll still find a way! Now if SHE truly wants to stop then you're 80% there! But to stop sucking her thumb you need to help her find something else to replace it with. For instance, if she sucks her thumb when she is tired or sleepy, maybe have a small blanket with a silky edge or a stuffed animal that she could hold on to when she starts to feel like sucking her thumb.
My 10 year old son sucked his thumb until he was 8. He only quit because he had to get an expander to correct the dental problems that he created because of the thumb sucking. During that time we used a beloved blanket of his to help him go to sleep. Then when the expander came out, he had a new "habit" in place and didn't feel the need to use his thumb any more.
Good luck! No matter what happens just know that there will be a time when she will want to change it!! (But I tell you, after the hard time that he had breaking his habit.. I made sure that my other 3 kids used pacifiers!! It is much easier to get rid of a pacifier than a thumb!!)
God Bless!! B.