I'd be interested in this too - my members have requested this at various points over the years, and if there's an active thread (that keeps floating to the top) that you're not interested in, it would be cool if you could just one-click ignore it. To view again, just go to user control panel and click 'view' link on the ignored thread/s.
luutruong has built this add-on and I've had the chance to test it.
It's working great.
Users get an 'Ignore Thread' option under the 'Thread Tools' dropdown when viewing a thread.
When you Ignore a thread then you no longer see that thread when viewing a forum or 'What's New'.
There is a new option in your account to view 'Ignored Threads'.
Here you can see a list of the threads you are currently ignoring. They are listed as if they were in a forum so you can view them and chose to un-ignore them, if you wish.
To begin with you can contact luutruong directly on here to purchase the add-on. It would be good to get some more users testing it so if you are interested, I urge you to do so. It's very reasonably priced
Thanks and massive thanks to luutruong for his work in putting it together.