I think Vespian pretty much hit the nail on the head with that reply.
There is a difference between mate-begging, as in someone coming up and pestering the fool out of you to be their mate, even when you've (kindly or more firmly) told them you aren't interested. If they persist or get rude, that much certainly can be a pestilence. If they bully you over it, or goodness forbid impersonate staff and insist that you are obliged to be their mate, take screenshots of this and report it. (This way something can be done about it).
However, when people say "stop the mate-begging!!!" to someone who simply asks, "Hey, does anyone want to be my mate?" once, this much is blown quite out of proportion, and is not rule-breaking at all. They asked a question, and are only seeking someone to roleplay with, rather than joining a large group. This said, ostracizing someone for searching for a RP partner in a large RPing community is quite absurd in my eyes, and so with respects to someone simply asking for a mate, there's no need to take action against someone who does this (so long as their behavior is within game rules).
When someone does ask, though, they just need to be kind about it--as all of us do.
Giving someone the benefit of a doubt and being kind is the best way to approach these situations. Give them your answer in a kind way, take it as a compliment, and move right along. However, if they do become rude, blocking and reporting will move the issue right along to the staff, who will gladly look into it on a case by case basis, as we do with all rule-breaking.