Suggested

Black List

It is necessary to create a Black List that will prohibit joining the league.
There is simply no other way to deal with clowns.

Please! 

Well as it's already known, the host doesn't own the league and can't stop anyone joining if they want. 
To stop people joining the host could always set up a password controlled league, without it the undesirables can't join.

If you close the league with a password, then no one will be able to enter. No potential newcomers to the league.

At the same time, there are people who constantly behave aggressively, insult other players and behave inappropriately.
No league needs such players, they only spoil the atmosphere.

P.S. Sorry for my bad English. 

Well the only other way then is to report the players to the mods and let them deal with it. 

Even with a blacklist button, as it's so easy to set up new accounts they will just come back using a different name.
Go password protected for a few weeks until the said players get bored and have gone away then you can remove it.

Whilst you cannot stop anyone joining what I've done in the past is just remove them and send them a message.
Bit like others have said just report them