Always keep the fingers clean by washing or wiping the bb hands with wet towel more often.
You may also try this:
My girl used to suck her fingers. Whenever she sucked her fingers, I used a clean handkerchief to cover her fingers. At first, she sucked the handkerchief as well. I kept doing this and finally she found it difficult to suck her fingers and decided to give up sucking both her fingers and handkerchief.