I tell them how I feel about them with an air of complete neutrality. If they're sensitive to the awkwardness which would presumably follow that sort of reveal, they'll probably leave of their own accord. If your partner is likely to be angry, then just get up and stand away. If they ask why you've done so, *then* tell them.
If they're upset, remind them that there's no such thing as being honest "to a fault", and that insisting on socially expedient white lies is morbid, juvenile, tiresome and cowardly.
You don't want to hurt anybody, but nobody can make you share the company of someone you dislike. End of story.