As long as one can only send messages to their "buddies" I don’t think it will become an issue. Well, at least to those of us who don’t accept just every person who makes a buddy request. Over the last couple years I have only needed to turn down maybe 4-5 members who looked to be spammers instead of someone looking for a dive buddy. Therefore, I only get messages once or twice a year from one member who runs a charter business in No. Calif. And I’m on the verge of Un-friending them since it’s obvious we will never dive together. Huh, just realized I have never un-friended anyone before. Will have to look into how to get that done.
As far as adding extra requirements or milestones you need to hit before you can send more messages, I think that will just turn people off and maybe slow the growth of the site.
Perhaps, you can add a feature to the site "Report a Spammer." You don’t have to take one member’s word for it. But once you hit say 5 members who complain, it’s time for Greg to addios them :-)
Rich