but you'd be relying on others to flag the rubbish content.

Yeah, and that's exactly what I'm looking for!

I mean, I know I'm not alone being tired of this behaviour. - I can't even chill and have a nice discussion in steem.chat or discord because I'll get bombarded with private messages. All asking for upvotes & follows.

I'm also nothing special at all. I'm far from being a dolphin or a whale, so just imagine how tired they would be of this nonsense.

I bet tons of people would use a feature like this, or a similar feature. - And that would bring this community even closer to each other. As you wouldn't be getting these nonsense messages and that would allow you to focus on the real messages you get instead.