Here's what you calmly and assertively say (practice it a few times so it becomes natural):

"It's been my experience that someone who can't intelligently add to a conversation and resorts to crude, or condescending remarks does so either out of insecurity and the need for attention or because he/she has nothing constructive to add - "

Which do you think it is (insert their name here and politely smile)?"

Then offer to make toast with him or her ;-)