I have a custom field, which auto-populates a job number upon converting a lead to a deal but the automation breaks if someone accidentally edits that field. I want to lock that field but keep all other fields open. Is this possible? 

I've tried through Setup > CRM > Modules > Deals > Record Locking Configuration but it's for record locking, not fields. You can select up to 15 fields to remain open in a record but it's not enough as I have way more. 

I've searched the forum but couldn't find anything relating to the issue. Any help on this would be greatly appreciated.