First off, You cannot delete a thread that you started. You can only delete replies that you posted.

I don't think there is a way to clean up your "my active threads" list. It's a function of the database which applies to everyone and cannot be changed by the individual user.

Why would you want to delete your original post? That would render all replies pointless. It would make things pretty confusing for everyone. Not a good idea.

If you want to see your active posts narrowed down to the last day or week, then do a search with your username in the search query and make sure you select the forums and timeframe you wish to search.