The Mists of Pandaria expansion released the Hardened Shell item, dropped by the rare NPC Nessos the Oracle in Kun-Lai Summit. This item has a 0.5 second cast time, a 40 yard range, and can dismount a flying player as long as they can be attacked (i.e. both hostile and PvP-flagged). It does not provide the victim with any sort of safety mechanism (eg. a parachute), which allows them to potentially die from the fall if they are dismounted from high enough. It also works on a Druid's Flight Form.
The Hardened Shell is currently the only way to dismount a flying player or to force a druid out of their travel forms.
The only other means of dismounting a player is with a Polymorph effect, namely the Mage's Polymorph and the Shaman's Hex. To clarify, these do not work on flying players or shapeshifted Druids.