It is easy top handle the roulette issue.

Make it an object you have to buy in the store for real money and don't make it cheap. Also make it lock the entire character to the level it is at for ALL jobs. Unlocking again being free. So Every time you want to stop exp you have to pay. That would probably stop most who want to use it just for roulette.

I for one would love a way to stop exp and have said so on these forums before.