K.H.
I don't think you are going to be able to stop her , this is something that she will grow out of as she get's older , does she talk to you with her thumb in her mouth? If so that could be affecting how she speaks and I would be firm with telling her 'no talking while the thumb is in her mouth' just like you would a kid with a pacifier. If she uses this as a security then there really is nothing you can do , my middle daughter used to have a blanket that went EVERYWHERE!!....it was filthy and I used to get major meltdowns when it needed to be washed , she is 4 now and has not had the blanket for over a year now.
Hope this helps
K.