Ours is relatively simple and effective. When meeting with a couple for the first time for dinner/lunch whatever and it is apparent that there is ZERO attraction for either of us, we do one of two things:
1. Start lovingly stroking the others back, which means "I have no interest". And we will start the process of ending the dinner/meet, etc.
2. Ask the other to scratch a sudden itch on our back, that means "Get me out of here NOW!". (We have had to use that one once, it never went past the first round of beverages.)
Whoever starts the gesture, it is the opposite who begins the closing of the meeting. That gives the one who does not to continue the clue that the other has picked up on the signal and then they follow up with helping to close the meeting.
Important to note though, we do not play on the first meet and that is understood up front by all.