Suggested

PM from friends only

I think the best is as an option in preferences, and maybe with the option to let you get only get messages from friends and people from your league too.

I understand why most players want these changes, but for me it'll be very inconvenient. I always send messages with a guide and league rules to new players or notify when I kick somebody off and in some other cases. So ideally it would be : 
1) Messages within the league should be always permitted
2) There should be a preference, but when a new player sign in this function should be off and can be activated later.

Yes, the default option will be to keep the setting disabled, and it will be the player's choice to enable it.

I agree with the blockage I get several messages even today the reason to write here is I am in favor of blocking private message for privacy reason would only be appropriate for friends sometimes I get ironic messages that annoys me a lot I am in favor of the block would help a lot in the privacy of the player.