That is a GREAT question. I try not to call to a single dancer--largely b/c often, calls don't even help this sort of dancer all that much. (Sometimes I call to them out of instinct, but I've been trying to check myself there.)
I don't realllly have a good strategy here except to enlist dance angels. If I've got experienced-dancer friends in the crowd (which I often do), I'll point out the problem dancer and ask if my friend will partner with them. (This isn't a favor I'd be comfortable asking of someone unless I a. am friendly with them and b. know them to be actively interested in partnering with less experienced folks and building up the skill level on the dance floor.)
Interested to hear others' take on this issue!
Cheers,
Maia