If your banneton shows signs of mold, the best thing to do is to replace it. Mold spores are fickle and no amount of scrubbing will rid the basket of them. Because of the nature of rattan, you won't be able to all the mold. Many people say to use bleach on the baskets, however you don't really want to use bleach on something porous that is then going to hold dough. Replace the banneton and store the new one somewhere free from humidity.