evilstepmom, you have my sympathy as we're in a similar situation but we're lucky that DS1 only sees his Dad a few times a year. DS1 is normally a happy, polite, well mannered, easygoing 9 year old. However, after every visit (usually during school holidays) he returns home with an attitude, all manners gone, deliberately defiant to see what he can get away with.
DS1's Dad is very unreliable, if we don't phone him we never hear from him. When he does speak to him, his Dad talks the talk, You're so important to me, you're the most important thing in my life etc etc but then never gets in touch of his own accord.
We've found that we have just had to say to him each time he got home, whatever behaviour that is acceptable when you are with your Dad isn't when you are here. And its reached the point now where he just snaps out of it, and returns to his usual self.
Its hard when my instincts are to protect DS from being hurt, to allow the infrequent, irregular contact with his Dad to continue but I feel that as long as we are providing a consistent family home life for DS I have to let DS learn for himself what his Dad is like. I never put his Dad down infront of him but it is slowly starting to dawn on DS1 that his Dad's words don't tally with his actions.
Be proud of yourself that you are being the mature party in this and have the childrens best interest's at heart by asking this question in the first place. Your stepchildren will learn to deal with this situation as they get older and even though you want to protect them and stopping contact could achieve this, i think its better in the longterm if they learn it themselves, confident that they do have you and their Dad to turn to as the ones who have always been there for them (thats quite rambling but hope it makes some sort of sense?!)