I would like to have the possibility to have a checkbox within the forms editor that we can choose to set some of the data fields to be set to read only or that you can choose not to update the contact specific records for some fields when someone changes them.

https://community.dynamics.com/365/marketing/f/dynamics-365-for-marketing-forum/360523/make-fields-read-only-in-marketing-form

There is a javascript solution but this does not work in all cases.