Probably, in a polite way, instead of declaration one can ask someone not to dance any more.
<Insert name here>, пожалуйста, не танцуй больше.
pozhaluysta, ne tantsuy bol'she.

If you want more of begging, please goes to the end.

And simply saying "You are a bad dancer" without noting that yes, not merely bad, but very, very bad, so much bad you are terrible, would also be more polite and go there. So,

Ты плохой танцор.
Ty plokhoy tantsor.

Of course, for female dancer you swap "плохой танцор" to "плохая танцовщица" (plokhaya tantsovshchitsa) and "танцор ужасный" to "танцовщица ужасная" (tantsovshchitsa uzhasnaya), because gendered forms.
Oh, terrible and horrible got a bit of sameness here, note.