I would say that you can take care of this by certifying a document before you send it out, not after it is returned to you. See here :Certifying and Signing Documents
Adobe sign has also a very good workflow that takes care of this.
However, if you're the creator of these forms, and you 're the one sending them out, you may configure on each signature field what portions (or fields) will become locked (or readonly) when a user applies their signature.
This only applies to certificate -based signatures.
You should add a Digital Signature field to the file and set it to lock all fields (maybe except for the Submit button) when signed. Then instruct the users to sign it before submitting the form back to you.