It might be too complicated, but maybe a 4 directions vote ? Up/Down : Agree/Disagree Left/Right : Makes the discussion go backward/forward This way you could

It might be too complicated, but maybe a 4 directions vote ? Up/Down : Agree/Disagree Left/Right : Makes the discussion go backward/forward This way you could express disagreement while acknowledging that the point is interesting, or like a joke without having it drown a conversation