Such circumstances can be avoided by not making the event public (e.g. on facebook or twitter) and avoiding talking about the event around a potential weaseler.
Weaselers can be identified by the lack of social life, obsession with one or more of the parties and an overall interest of the event.
A weaseler can be identified by their hand gesture. The back of the hand is placed under the chin with the fingers facing outwards. The weaseler then moves the fingers in a waving motion such as tentacles flailing from the body of an octopus.
Boy 1: Hey man want to come to the shopping mall?
Boy 2: Yeah man, fo sho, what time?
Boy 1: How about 2?
Creep/Weasler: Hey guys, what time is it again? and where are we meeting?
Boy 1: No creep. You can't come. You are such a weaseler.
Girl 1: I had a party last weekend and that creep just showed up.
Girl 2: Really? What a weaseler.
Girl 1: I know. He is such a grish.
Girl 2: One time I was on a double date and he invited himself and a girl. He performed the ultimate weasel.