My sister sucked her thumb for a LONG time. I think there's something out there called "thumb" to put on the thumb that taste's nasty. I think the pharmacist might have it behind the counter.
My oldest daughter (who's almost 6) had a pacifier (until she was at least 5, I think) and only wanted it if she was really really tired or upset. Once she'd fall asleep, she wouldn't want it anymore. I wasn't terribly concerned about it because she hardly used it. BUT one day my husband told her it was bad for her teeth, and she quit. I asked her one night if she wanted her nuk, and she said, "No, it's bad for my teeth." BUT she did have it in the drawer of her nightstand.
For what it's worth, and good luck!