I said it many times and I'll say it again: Please consider the friends-of-friends!

Have paid users. These are tier 1. Their follows are tier 2 etc. Put limits on each tier independently. For example:

tier 1: Just like all the other paid relays

tier 2: No more than 5 mentions per minute. No more than 20 events per hour or 100 per day. Events get deleted after a month.

tier 3: 5 mentions per hour. Event gets deleted after a day. No back-dating. No links. ...