I use "locked" the same as "read-only". I used the properties box to have the first signed digital signature field to mark as read-only most previous fields as well as the first signature field. The second did the same, including locking all signatures. The requestor simply cleared his signature and altered the document, while the approver's signature was still in place. I couldn't figure out how they did it as I thought the approver's signature would lock the requestors by making it read only.
My first idea to fix it is for fields to not be locked with the first signature but with the second. That way removing it would not show our approval. I'm open to more ideas. I'd still like to have the prior fields read only with the first signature and that signature not removable with the addition of the second.
So, you are manually setting "read-only" flag on some form fields in a signed PDF. This does not achieve what you want to do.
Read-only flag only limits interactions with a form field that change the field. Other interactions that do not change the field are still allowed. Signature field is just a container for a signature object in PDF. When you clear the signature you do not modify its filed but rather change the signature properties (in the latest versions of Reader you can see deleted signatures in the Signature Panel). Thus, clearing a signature does not actually modify the signature field itself which is what "read-only" flag prevents, and that's why it is possible to clear a signature contained in the read-only field.
The proper way to achieve what you want is to check "Lock Document After Signing" check box in the "Sign" dialog when the last signature is being signed (approver's signature). If you do not have control over what approver does at signing, you can apply one more your own signature after the approver's signature and check "Lock Document After Signing" check box during the signing process. If you're interested about details, locking document uses PDF's FieldMDP mechanism which is described in the PDF Reference.