I just unfollow anyone who does that. There are quite a lot of people who just stay “on topic”, more than enough to fill a feed. So while I might miss some interesting content from people I otherwise unfollow, it turns out a lot of the time someone else retweets it anyway.

Unfollowing itself when you see something off topic takes very little time.

I do agree with the other reply that being able to subscribe to topics from people would be awesome, but in the meantime it’s pretty good if you maintain a zero tolerance policy for off topic stuff.