I know there is nothing in the game rules just now to stop other alliances messaging members of another alliance but it is becoming a common issue now that members of the alliance I'm part of are experiencing on a daily basis.
It is beginning to annoy them that they are being sent these unsolicited messages and they are asking for a way to prevent people contacting them directly unless they are either part of the same alliance of are friends with the other person.
Surely the devs can introduce a way to control this.
As to providing a place of contact for new applicants that could be done by having a button that says contact alliance admin and that will send a message to alliance admin (not co admin).
Yes blocking works but only works after the fact.
It is about time the devs changed this so that we can decide who can contact us in game.