There's a SMF mod that does this but it's not compatible with the 1.1.10 version we seem to be running here.
IGNORE TOPICS v1.0
By Karl Benson
Compatibility
For SMF 2.0 Beta 3 Public, 2.0 Beta 3.1 (and above)
It will not work for ANY earlier versions.
Introduction
Adds the ability to ignore individual topics for the purposes of RecentPosts/Recent/UnreadReplies/Unread sections of SMF.
Ignored Topics will still be shown as usual on the BoardIndex/MessageIndex and the topic itself viewable as normal.
Neither will it affect any notifications.
You will need to enable Quick Moderation to use the feature. (which enables the checkboxes)
Features
o IgnoreTopics Profile Section
- Select to Un-Ignore topics
- Un-Ignore All Topics
o Alters RecentPosts/Recent/UnreadReplies/Unread sections of SMF to enable Ignore Topics
o Use the Check Boxes
o Supports Languages
- English/English-utf8
- English_British/English_British-utf8
http://custom.simplemachines.org/mods/index.php?mod=1193mystic_1