Hi Karin,
I feel ya on this one. Although my FIL does not use bad words in front of our little one, he is a very negative person and a rather free spirit...he says what he wants. When the baby was very little, he started saying things to her about her "No good Grandmother" and not to listen to her mean mother. I corrected him and he got very upset and asked what was wrong with that because he was joking and he could joke if he wanted to. I tried to reason with him, but he didn't want to hear it.
He started out with a lot of visits, but with my strong personality and his do as you please personality, it is not working. He no longer visits on a regular basis. I really wanted my little one to have a good relationship with him, but I feel there is enough negativity in the world without it being taught in the home. I tried to tell him I would never teach her anything negative about him, however again, he didn't want to hear it.
I also ran into a problem in the preschool parking lot. A mom was yelling at her kid telling her to sit on her @$$ and shut the #&!! up. I kind of yelped and said, "Hey I have a little one out here" but she kepted yelling at the poor kid (probably a 6 year old). So I mentioned she might watch her mouth around other peoples kids and she yelled at me too. I could care less that she yelled at me, but atleast she goes straight to her car now without all the garbage coming out of her mouth.
It is your child and if you have to, I would just cut the visit short. They may not even realize it is something that bothers you and a simple reminder is all you need. As well, many people think that at 6 months old children don't know anything...how wrong they are.
Best of luck.
C.