professional mold removal can be quite expensive. ...

try to treat the surface with bleach and see if that helps.

i've heard that if the mold is particularly persistent, it's possible that the mold has gotten into the pores of the wood underneath. in that case, you need to consider removing the paint at the spot, apply an anti-mold coating, then re-apply paint -- a laborious process. however, i have not seen this done in person so i can't guarantee the validity of this.