Hey John,
If you mean "hide" threads you don't want to read from your forum display - no, it's never been a feature.. If you mean stop getting e-mail notices from threads you are subscribed to - you can do that either from the User Control Panel or at the bottom of each e-mail sent out (at the footer of the e-mail). Just click those links to unsubscribe from that thread or even all threads you are subscribed to...
Other than that - the only other way to "block threads" is simply don't click on them to read them??!!
Best to you John!
John