I saw an old thread on this, but it alluded to an old version of Vanilla. Is it possible to do this in 2.1.8 or would it scale badly?
In order to make it scale best, I would create it as an additional css class. Users can choose to "Hide from Recent" it instead of ignoring it. Discussions marked as hidden just get the class .Hidden and that's it.
You might want to add a "Hidden Discussions" counter on the "Recent Discussions" page so if a user decides to ignore/hide 100 discussions, he might only see a blank page. So he should be informed that there are X hidden discussions on the current page. If you make this a link to a profile setting page where he can unhide the discussions, you would have a nice plugin!
Do you want to create that?
Could you describe what you mean by ignoring?
If not receiving notifications or marking the discussion as read without opening would suffice, take a look at these 2 of my plugins:
The first one would also be a good base for implementing what @R_J described.
My themes: pure | minusbaseline - My plugins: CSSedit | HTMLedit | InfiniteScroll | BirthdayModule | [all] - PM me about customizations
VanillaSkins.com - Plugins, Themes and Graphics for Vanillaforums OS