I want to make it so that each user can only be assigned once to a specific field across all entries. For instance, let's say we have a bunch of teams. A user can only be on one team at a time. Once assigned to a team roster, they should not be available to assign to another team. Is this possible?
Could you not come at it from a different angle and instead choose your team by editing the user? That way you can assign a team field to the user and restrict that field so that you can only pick the one team.
Then in your templates you can use relations to link them all up.