Stop thumb sucking?

My 15 month old sucks her thumb. Which I really don't have a huge problem with in itself. The problem is she sticks her thumb so far in her mouth, she has developed this bad blister looking, really red patch at the base of her thumb. I lather it with lotion, but I am convinced that she must stop sucking because I know that it must be painful! How can I get her to stop safely? (I refuse to use any hot sause, so please don't say that -- remember she is only 15 months old.)