Why not just change the option so they do not send you any job emails. Deactivating is permanent. Down the road, this company could get things together and get a high value company that you will want to shop.
Once you deactivated, you have pretty well burned that bridge and cannot ever work for them again.
Why on Earth would you want to deactivate a whole scheduling/reporting system? Prophet is not an individual MSC. And who knows? One of those 10 companies that you want to keep working for could change to the Prophet platform in the future and then you will screwed. Does not make any sense to me.