No one ever forbade him from using it. It's just that he doesn't '''want''' to use it if it's only for his own benefit.

it is not the sword it is the swordsman

Its not mentioned in the anime but in the LN, also posted in the wiki, "Kazuto resolved to never use Excalibur for personal gain as he believed that the sword contained the «caliber» of himself and his companions." The author, Reki Kawahara, even stated that if Kirito was to use Excalibur in their duel, he would have had the advantage because of the swords stats. But, seeing that Kirito would never use that sword for personal gain makes this "what if" scenario pretty meaningless, imo.