[bardan]

I'm talking about a big red button "spam/harmful" rather than a personal preference list.

To be a little clearer: you have your own private filters and a separate, distributed block list shared amongst your friends, which is for bad content. You use the big red button when you encounter spam or harmful content, and this bad content/user/node/etc is added to the shared block list.

You initially trust all of your friends to use this block list properly, but if you find one of your friends is misusing it you can revoke trust from them and ignore their contributions.