You could add a "parallel" list (PL) linked to your current list/item, and on the PL add the fields you want to "protect" as READ-ONLY, then you could manage the related items on PL by settings permissions on the item.

I hope you have understood my idea

Hi Fernando,

Thank you for your kind suggestion. However, I would like to make the field read-only in the same item not in a different list.

Thanks,

Hi,

If you use Nintex Forms you can add a rule on this field and if this field is not empty, just disable it

The result will be that whenever you fill the field once, your field become disabled!

If you don't use Nintex Forms, I don't see any way of doing it but Javascript!

Hi Alex,

What is rule should be used to disable a field section after it is filled.