(Thanks for the typo.)
The best method to deal with this scenario is to keep unwanted people outside of a protected perimeter.
Yes.
My complication is what if the unwanted people actually could become wanted, needed skills to benefit all?
An idea is to start out before the SHTF and form alliances that will ensure mutual defenses / increase chances of survival, when it happens, taking into account the last resorts i.e attack first / defend what is / escape plans being in place.
The more like minded people, the better the chances of success / survival, the more new people would want to join, instead of deeming them unwanted. Unity Create Power.
Just a thought.