It might be better to make the posts visible to your followers, it's probably much the save as what you are asking for, but might be an easier feature to implement.
In the meantime, I think that your only option like a lot of us, is just to ignore, these people can't be stopped and are unfortunately the scourge of Steemit :-(
Yeah, that might be a good thing to actually. To make their content only visible for their own followers if they get reported for this spamming-behaviour. - I like it. As long as something is done instead of ignoring this whole issue I'm totally fine with it.
I mean, today, it's fine. We can handle it. - How about 6 months from now or a year? - It will be crazy if you don't work to prevent it.