I don't believe they need "protecting" in the first place, any more than they need protecting from the possibility that other players might choose to do a bunch of missions for them and get them into a conventional BGS expansion.
Since 2018, all systems have been independent of each other, so being present in an extra system largely only creates extra maintenance work for the player group if they choose to adopt the faction's presence in that system too. Just ignoring it is an option.
If they really don't want to be there, they have the option to set up and lose a control conflict in the new system, and then either set up a Retreat (these aren't going to be high-traffic systems out on the fringes of the bubble), or just ignore it from there on the grounds that they'll probably retreat eventually through random movements if they don't control a station.
Or they can just take it as one more system on their list that they didn't even have to work for and no-one else can claim "but we were there first" on either, and keep it.