I keep reading here how it's "so simple" to just say no and not invent any excuses. Well, I decided to finally go out on a limb and try this last night, when I was asked to dance by an old guy who also seemed to be missing some teeth. Yes, I totally rejected him because of his appearance. Typically I would have said I was getting water or one of the many other reasonable excuses. Instead, I said "No, thank you" politely. He kind of threw his hands up a bit as if exasperated / upset (not a huge gesture, but it was obviously not a smooth interaction). So I think that was a fail and I'll go back to my excuses.
A no if said kindly (don't have to be necessary with an excuse) will be accepted the majority of time. It may hurt the person feelings for 30 sec and then he will find another partner and will forget about it. If you say no a second time he will understand and won't ask again. We are all adults and an adult should be able to take no as an answer or that persons had some issues in terms of parental education. If you honestly think you said no nicely to the toothless guy, that's his problem how he handle it.
The constant no with lame excuses or pity yes with passive aggressive attitude behind or even the absence of answer as I experienced it recently in Croatia (in the bachata room if I remember) doesn't elevate anyone.
I personally observe how women reply to demands whether it's a yes or a no and their attitude during the dance and if I don't like what I see I just don't ask.