1st- You delete the Tags with Mp3tag from your Music File
2nd - You play the Music in WMP
3rd - When you open the Music File in Mp3tag, the tags (you deleted in step 1) are back in the Music File ? Correct ?
If so, WMP has it's own database, so the tags are stored in both the Music File and the WMP database. What you could try is
a- Delete the tags with WMP and see what happens
b- Delete the tags with Mp3tag and have WMP rescan your Library.