maybe speak to your step dad whilst your mum is present and say he really gives you the creeps, is it possible that he can just deal with this guy on the business premises or at a cafe but not at home. If the guy drops past when your step dad is not around, definitely just say as Gretch suggested, and say you'll let him know he called around. Don't open the door the whole way or allow a gap where he may invite himself inside, and don't feel like you have to make friendly conversation - just go to the point, and start closing down the meeting, and the door (but with a smile). If he ever is at the house talking with your step dad, invite your mum to go for a little walk or have a coffee in a cafe, or something. Stepdad will soon get the hint that this guy is not wanted at the house if he is the only one who has to entertain him.
