This is fine because they all agree each other and do such party farming - nice cooperation.
But they of course can't block other players - URLs are public, everyone can join and nobody can limit anybody.
Even if they "fill" whole URL room it doesn't mean they all will take part in same URL tournament - if more players join room, the system will choose 8 (usually) random players for tournament.
If such thing happen they can't force other player to give win etc until he agrees on his own.
So, if they are lucky enough that they are all friends and they all managed to take part in same URL tournament without other players - good to them.
But they of course can't block other players - URLs are public, everyone can join and nobody can limit anybody.
Even if they "fill" whole URL room it doesn't mean they all will take part in same URL tournament - if more players join room, the system will choose 8 (usually) random players for tournament.
If such thing happen they can't force other player to give win etc until he agrees on his own.
So, if they are lucky enough that they are all friends and they all managed to take part in same URL tournament without other players - good to them.