Yeh, I know what you mean. I have a molly that's a very big bully. I was ready to take her back to the lfs -- but didn't want others to have to deal with her either. She's constantly harassing the other female who is a real sweetie.
Today I took her out of the 20G and put her in a 5 gallon all to herself. You should see the other molly now (the sweet one) -- happy as a clam -- swimming all over the place out in the open instead of hiding in the cave.
This is the second female molly I've had that's been a total wench towards other fish. I thought they were peaceful....