As I recall, my mother usually discards the wine that is spilled at the Seder during the listing of the plagues. But I think she does this because, well, just because it's spilled wine and it is not worth salvaging.
But my wife's family has a strong Minhag to make sure the wine is discarded, and either a separate plate or a napkin is used to ensure it doesn't get on the food plate. The reason I was given is that, after the spilling, the wine is associated with the plagues, and so it is "cursed" (either literally or equivalently, I'm unsure).
Is there a source for this idea?