Flying spells are one of a group of very powerful spells. The general idea behind all flying spells is that they allow you defy gravity in one way or another.
These spells must be used with extreme caution, for if they are cast too powerfully the target may never return to Earth, and if they are cast too weakly one will not have the power againt gravity which they are expecting.
There is no way to reverse a flying spell. The only thing you can do is wait for it to diminish over time.