How do I un-link a CTO Job from my Sales Order?
A job should only be linked to an SO when the item is marked as CTO (or one-off) and for these type of items that means there should never be a job without a hard tie-back to the SO, because both of these are expecting to have changes made to the default specifications based on customer input. If there does not need to be customer specific changes, then it is best to have the item not marked as CTO or One-off, but instead run with just a zero Reorder Level if you only make quantity when you have demand.
To specifically answer the question, the only way to remove a hard-link is to delete the job which then deletes the CTO field on the SO. If you want the job to exist without the link, then you should ensure that the item is no longer marked as CTO and then generate MRP for Net Demand and MRP will suggest jobs based on each non-CTO item's supply and demand with it's planning period.