I have many files that WMP tagged with it's PRIV tags.
I have tried several ways to remove these tags with no luck.
I created an Action Group, Chose Remove fields as the action and entered this for the fields to remove:
WM/MediaClassPrimaryID;WM/MediaClassSecondaryID;WM/WMContentID;WM/UniqueFileIdentifier;WM/WMCollectionID;WM/WMCollectionGroupID;WM/Provider
In Mp3tag do a "Tag Cut" and then "Tag Paste". The cut will remove the tags and the paste will only put back tags that are supported by Mp3tag and since Priv tags aren't supported they won't be copied back.
Make sure in Options > Tags > Mpeg you have ID3v2 checked for both Write and Remove.