It can't. And this is something people can do now and will still be able to do if it goes open only. Unfortunately due to how cheap alt accounts are during sales, it opens up the opportunity for this sort of behaviour even more. Its one reason FD probably can't ever contemplate significantly rewarding direct PvP. Just too many possibilities to exploit via either collusion with other people or by using alt accounts.
Of course, if everyone is in open there is a chance of a random encounter with an opposing player who could mess with the plans, but considering timezones, instancing, etc, you can probably do whatever you want in peace most of the time.
Perhaps FD need to implement (if they haven't already) some sort of monitor for people running multiple accounts from the same IP. But then, they would need to watch what is actually happening as well, since people can be running multiple copies on the same IP for family and friends or just in a shared network space like a student dormitary. It would put a lot of overhead on FD unless it was just raising flags in very specific situations. And then, that only helps with people running multiple accounts, not collusion.