The underlying problem in any game economy is the introduction of currency due to activities in the game. The solution to this problem is known as a gold-sink, the removal of virtual currency via other activities. One gold sink is crafting, but it isn't effective enough as a gold sink due to the rapid introduction of zeni. Other gold sinks are guild donations and registration fees on the AH. The quickest way to stabilise the economy is the introduction of a gold sink which rewards bound items which would originally be obtained from the Cash Shop. This would reduce the demand for Cash Shop items while also removing Zeni from the game. This would be introduced most easily in the form of a capsule machine similar to the pet vendor, but with different items and a (much) higher zeni cost.
For more information on a Gold Sink, view this wikipedia article:
https://en.wikipedia.org/wiki/Gold_sink