There is
this mod which will allow you to make RSS items as posts in a single thread instead of a new thread each time, but honestly it is a bit on the difficult side to install needing manual file edits. If you can live with them as new threads it will make life easier.
As for the two week thing, I could have SWORN I saw an "RSS Pruner" mod that auto deleted old RSS posts after a set time but I searched and searched and can't find it here anywhere, might have been removed.
Another possibility is in forum manager you can set the default thread age to "2 weeks" and it should hide threads older than 2 weeks for most people viewing- but that would affect every thread in that forum, not just RSS threads.