from what you said i assume you got contact info from the person you're not interested in making out with?
if this is the case i don't see a problem with contacting them btwn now, and the next PC, just to say hi, and working up to the whole "hey let's not make out next time" thing. hey maybe by talking with them you will want to make out
also, there shouldn't be a problem with you honestly communicating with them, considering honesty, and communication are some of the core principals of polyamory. if there's a reason why you don't want to make out tell him. if there was no spark tell him, if things went too fast for you, tell him. etc.
p.s. unfortunately i won't be at the next PC, but maybe we can meet at the one in March?