Why not just make it so that if someone switches to a job outside of the specified range, they receive no sp/reward? Does there really need to be an elaborate lock-out system?

To clarify, if the behest is r10-20 range and you switch to from a r12 job to r11 it's still fine and you receive sp when mobs are killed. But if you switch from r12 to r9 or r21, the mobs killed during the time you were on those jobs will net you no sp and you wouldn't receive the reward at the end.