I say, "Sorry, I have already accepted this man, but if you like I'll save the next waltz (or whatever) for you". More importantly, dance etiquette requires you to sit out a dance if you refuse the first man who asks. So if I don't want to dance with the first guy, which is certainly my right, I sit out the whole dance. Don't be pressured into dancing with someone you don't want to dance with. Some guys act like if they ask, you must say yes. I even had one man yell at me when I politely turned him down.