I found this on a website: http://www.stopbitingnails.com/Thumb_Sucking.htm
Not sure if any of it will work or not but hope it helps.
1. If your child has a favorite chair or place they like to suck their thumb, put a "TV chair" near the TV instead. Move the place they normally watch TV.
2. Be aware your child may have a favorite toy or blanket they like to suck with, and move it to a place where your child can see it, but not have it to suck with.
3. When you do not see your child with their thumb in their mouth, tell them how pleased you are about it! Reward their efforts with positive reinforcement, kind words, and even surprises.
4. Think of things you can do together instead of sucking (remember, many children suck out of boredom), and do them. Keep a "busy box" of manipulatives nearby which will keep little fingers busy and out of the mouth such as Play dough, Koosh balls, Silly Putty, Legos, crayons and markers, books, stress balls, lanyards, beads, etc. Bring them in the car and place them by the T.V.
5. Limit T.V. usage, if this is a trigger for your child's sucking activity. Suggest a bike ride, computer game, or talking to a friend on the phone instead.
6. Never hesitate to consult a professional if finger sucking persists past age 4 1/2 - 5.
7. If a pacifier is a problem, don't promptly withdraw it's usage. Gradually wean down the daytime behavior, ending with weaning of the nighttime habit. Prompt, sudden pacifier withdrawal may promote the usage of a convenient thumb instead.
8. Before the age of 5, children are seldom ready maturity-wise to understand the necessity of why a thumb habit should be curbed, and therefore, it is best not to make it an issue until they are truly ready. Otherwise, they may resort to more continuous sucking as they become frustrated in their attempts to stop, seeking even more comfort from a convenient thumb, engraining the finger habit even further.
9. If your child over 5 expresses an interest in stopping, encourage this, utilizing the services of professionals such as orofacial myologists, or dental specialists, as needed, and depending on the method of therapy you wish to pursue.