D.,
My son (almost 8) went to a dentist and he has a retainer in his mouth that actually prevents him from sucking his thumb. (It's about 300.00)Although it seems to me like he replaced it with picking his nose!!!! LOL!!
As for the @ home version, try painting her nails it makes it taste bad enough not to suck (don't tell her why you are painting her nails). You will need to put on new coats to make it stay bad tasting. It's a good way to help stop but not let her feel "pushed".
You could also try carrying around hand sanitizer. I used to tell my son that his hand was REALLY germy and not to put his thumb in his mouth, until I gave him the cleaner or he would get sick. Once on he didn't want to suck his thumb. He then only sucked his thumb at night. Obviously not completely fixing the problem.
Thumb sucking is a hard one to stop - it's not like you can take a thumb away like you can a pacifier.
Good Luck!