Well, that second problem is a Windows thing.
Windows does not want you to simply unassociate files. It wants you to switch associations. If the file already has an association and you remove that association it's like making it an orphan. So in order to remove an association from PSP you have to associate the file type with a different program, essentially giving it a new parent.
However, that is not the same as excluding a file-type from Manage mode. So even if your problem was similar, which I want to say it is not, it wouldn't have changed anything. You would have to do what xfsgpr did and remove the offending file from the reach of PSP.