My roller hockey league has 8 different divisions, and most people keep coming back..so the guy who runs it generally knows where people belong. There's kind of an un-written rule that the lowest division is allowed 1 higher division player per team (within reason, can't have D1/2/3, semi-pro players playing in D8..and D7 players in D8 are no big deal).
Teams need to have subs listed on their roster before the 3rd game of the season, and have them approved..so there's no sub-ing in ringers.
Players must play 5 out of 10 games in order to be eligible for playoffs, to prevent teams adding a guy who doesn't pay part of the team fee..just to use him for the playoffs.
Nothing can be done about a new player joining a team that no one knows, and having him be too good for the division. But he won't be allowed back in that division the following season.