Disable editing of some fields after filling in

Hi there

I know it is possible to select the option to have the user not edit a field, but is it possible to create a field that a user can edit but then not again after they have filled it in.

I am setting up a warranty registration process and I don’t want people to be able to change the Date Product Purchased, for example, after they have filled it in so that they can’t cheat the system.

Any ideas?

Thanks!