Enclose such strings in the grave accent (top left to bottom right).
If the data in MP3tag is as you expected and the player shows stray data ... then, I am afraid, there is not a lot that MP3tag can do about it.
It may be worthwhile to scan the files for consistency just to make sure that there are no double or stacked tags. See the Howtos to see links to programs that do such scans.
If you are still not satisfied - could you supply a file that shows the wrong genre in the player?