Hi,
Did you build your plug-in using the Acrobat SDK as the template? If so the SDK was created when Acrobat allowed anybody to delete a signature. Acrobat itself has since been modified because of customer demand to check if the user has access to the private key that was used to create the signature and if they do they can remove their own signature, and only their signature. If you don't have access to the private key then you cannot remove the signature and that's the behavior you see today using the built in Acrobat functionality. My guess is the reason that your plug-in allows anyone to delete the signature is the SDK didn't (and still doesn't) provide you with a call to see if the user has access to the private key. That's something you're going to have to figure out on your own because the Acrobat HFT call is private and third party developers like yourself don't have access to it.
Steve