Pretty easily. If they're a friendly, affectionate cat, simple instruction and reinforcement can work pretty fast.
If they're an unruly pest of a cat, you have to make the desk uncomfortable for them to be on. Sometimes permanently.
If they're the kind of cat that only listens when you're around, it might have to be a bit of both.
General rules:
Exceptions may be made at the discretion of the mods.
Pretty easily. If they're a friendly, affectionate cat, simple instruction and reinforcement can work pretty fast.
If they're an unruly pest of a cat, you have to make the desk uncomfortable for them to be on. Sometimes permanently.
If they're the kind of cat that only listens when you're around, it might have to be a bit of both.