Validation or Workflow to disable changes to certain fields (field level validation not possible)

I have an Object called Demand. It acts like an intersection table between Accounts and Locations. i.e. each record will have the Account and Location lookup fields and certain other attributes.

What we need is that once the record is saved with the Account and Location populated, they should not be able to change it to some other value (only those 2 fields others can still be edited.)

 - I do not want to use a dynamic layout for this.

 - I cannot use field validation since the fields are greyed out.

 - Can I use a "Before modified record saved" workflow with cancel save for this? what would be the condition for this?