Thread Display Option

[Grusbus]

I need help with my thread options.

I have a buy/sell thread that I need to have a time limit, the post should not be displayed more that 3 months (http://fiatclassicclub.se/forums/forumdisplay.php?f=6).
Know there is an built option for that but all users are able to change it back to showing all post again, this section needs to be blocked at 3 months and only admin or moderator to change the time limit.

Anything will help, thanks