Making rate field read only

I have a legal requirement no make Sales Taxes and Charges read-only in the taxes field of a Sales Invoice. I have successfully made read-only other fields but Rate (the most important) has stubbornly refused. I am using Customize Form method but I also tried this: How to make child table field read only?

Could this field be special?

Did you try Customize Form for “Sales Invoice Item”?

No I don’t need to customize Sales Invoice Item. But as a test , I made the Rate field on Sales Invoice Item and it works. Still no luck with the same field on Sales Taxes and Charges