I have not reviewed thew API documentation, but I can comment...
1) To "clear" or remove a digital signature, you need to have access to the "private" key (digital credential\certificate) that was used to generate the signature. This means that typically, only the signer can remove their own signature.
2) You cannot sign a PDF document with Reader, UNLESS the PDF has been "reader extended" with the "signing" permission that will enable the ability to sign a document with Reader. A PDF can be extended with LiveCycle Reader Extensions (server product) or with Acrobat Professional. I have nottried it myself, but I would assume that the "signing" permission would need to be enabled on the document to allow Reader to remove (clear) the signature, assuming agaion that Reader had access to the private key used to create the signature.