I suppose you are referring to the MRU list? I'm going to add a context menu for each item, with options to remove it from the list or remove all.
Meanwhile, you may use the Windows registry editor tool at the "HKEY_CURRENT_USER\Software\PDFShellTools\PDFTools\Stamp\RecentItems" key to edit that list. Make sure the names of the items you want to keep are numbered in order, i.e. str0, str1, etc.
Or just move to another folder or rename, temporally, the saved stamp files and run the tool.
