Dynamically Making a Field Read Only

Hi

 I am trying to make a field Read Only after a value is entered into it.

Based on inputs from earlier community posts, I tried the following but, it does not work.

NOT(([<ContactType>] = 'Referral' AND PRE('<IndexedShortText0>') = [<IndexedShortText0>]))

Here IndexedShortText0 is the field I want to make Read only.

I entered this expression in the Field validation property for the field.

Any help is appreciated!

Thanks

-Sooraj