Alexey
The blacklist is also needed in leagues
Yes that would be very useful indeed in banning repeat offenders/those that do not respect the rules and basically just create a toxic atmosphere within the league.
My only concern though is that we as humans tend to misuse power and this is especially true for league hosts that lack experience. These types of hosts could actually be the ones to make the game toxic for players if given such a feature.
I would modify your suggestion that mods be given the power to ban/blacklist managers from the leagues after proper investigations have been made. That would be fairer imo.