How to control who messages you

Is there some setting somewhere where people – and I'm talking about admin particularly – can automatically block messages from people who aren't on a friends list or a follow list or something?

It's not that I don't love the members of our forum a lot – it simply that it gets a bit too much receiving emails from them all the time asking for advice which is available on the forum. If there is not an inbuilt setting then is there some third-party product which will do this?

Thanks