Not able to disable Sales Order Header Text Manager

Dear All,

I am facing a strange issue with LN 10.7 Cloud version to disable the text field(tdsls400.txta), So Header text should be disabled from front end and no user should be able to update it manually.

I have tried lots of ways to do the same but nothing worked for me, please help me if anyone came across such kind of problem. Below are the things which I have tried till now.

table extension - used read.only to disable - not triggering when updating text.
session extension -
text.manager- before.command - disable.fields("tdsls400.txta"),
set attr.textmode = 1
under before.display.object used disable.fields("tdsls400.txta")

Even after tried everything I thought to put a validation so when user change the same it would popup error messages and not allowed to edit. but still no luck.

table ext - make.valid - not triggering
before.save - check the old text and try to return dalhookerror, its giving error when saving record but text updated even after selecting do not save.

Any help on this will be really appreciable.

Thanks