This feature is one I get requests for too. Since logged-in users presumably don't get cached pages, I'm not sure how much of a concern performance would really be. For displaying latest posts, it would only involve adding a few WHERE clauses to the relevant DB query, you wouldn't need some kind of JOIN statement.

When displaying the topic itself, it could be done on the client side in JS+CSS, e.g.

.post-wrap[data-userid="xxx"] { display: none; }