User blog comment:Omega natsu2/Fairy Tail Episode 197: Hisui Rises, Review./@comment-24622358-20141108234441/@comment-7418318-20141109081106

Loke was temporarily lifting his contract with Lucy before he had her permission though, so it would seem that at least some Spirits can do that.

It's also a possibility that all Spirits could in theory break their contracts, but they are forbidden to do so by the law of the Spirit King and would be sentenced to death by exile if they did. With the King out of the picture for now, the Spirits can get away with whatever they want.

But putting that aside, the keys Hisui made really make no sense for me either. They only work once a Spirit has lost their will to fight. But is there even any need to force the Gate closed once they lost the will to fight? So unless they'll have some other use that Hisui either didn't know about or was unwilling to give away, they will be completely useless, even if they work in the first place.