Frankly, in the "Show new replies to your posts." which, like almost everyone
else here I use as a "list of threads I'm actually interested in", there are some
old threads that I've entirely stopped to care about, but refuse to die.
How do I get rid of them ?
You can't.
SMF is gay and only allowed for that as an "upgrade", which Theymos refuses to upgrade to. The best way to ignore a thread is to troll it so that no one will talk there.