Our league is Private so the Commissioners are in charge, not Yahoo, but I think we can all agree this is a reasonable standard to follow -
Yahoo fair play rules -
Yahoo requires all managers agree to some basic rules to keep things fair.

Don't engage in any collusive action.
Don't drop or dump players from their team with malicious intent.
Don't take any action where its purpose would hinder fair play in the league.
Don't make roster moves that will intentionally hamper the play of other managers.
Penalties for rule breakers
Yahoo won't remove teams from a league post-draft, however, we will restrict the offending manager by suspending aspects of their game, including:

Some or all league privileges, such as trading or editing their roster.
Deactivation and removal of their Yahoo Account from all leagues and Fantasy games.
Any other action that Yahoo, at our sole discretion, feels necessary to restore fair play.