A user can only clear a signature that was applied by that user. Because Acrobat on this re-imaged system does no longer know that the old user is also the new user, that option is grayed out. Unfortunately, there are not too many options you have at this point.
As a user with a subscription, you can download a signature stripper plug-in from PDFScripting.com - if you do have a subscription, log in and then use this link: http://www.pdfscripting.com/members/programs/fileinfo.cfm?ID=94&action=Display - if you don't, a subscription is unfortunately not free.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com