ITF World Tournament Rules as of 2013 do not use belts as official categories in sparring. Per section T31 - Divisions:
(A) Male and Female groups.
N.B.: In individual sparring matches competitors shall participate in the category according to their weight. It is not allowed to participate in any other weight class.
Honestly, since belts are arbitrarily awarded by instructors, this is pretty much the only way to handle the situation. Your individual tournament might have their own rules, but we can't comment on those rules because we can't see them.
If you're discussing special technique, the World Cup rules group the colored belts into a single group, so there's still no conflict here unless your individual tournament operates differently.
- Coloured Belt (Blue and Red Belt in one division)
- Black Belt - 1st - 6th Degree
For forms, it looks like it's broken out a bit differently, but starts at 4th Gup:
- 4th Gup will compete with the Optional Pattern being any one from CHON-JI to
- 3rd Gup will compete with the Optional Pattern being any one from CHON-JI to ToiGye,
- 2nd Gup will compete with the Optional Pattern being any one from CHON-JI to
- 1stGup will compete with the Optional Pattern being any one from CHON-JI to
As regards disputes, the only rules I've found are for sparring, and require the coach to submit a written dispute. Again, your tournament may have its own procedures. Again, I don't think that the rules really rule out someone competing outside of their belt range since, again, ranks are arbitrary. It looks like the belt rank merely decides what form, or range of forms, will be performed.