You can flatten it: http://www.uvsar.com/projects/acrobat/flattener/
and then add standard password security.
Or you can digitally sign the document which can lock it from further changes. This is what's done when you save as a certified document in Acrobat.
Bear in mind that with Acrobat, even a flattened file is editable (that's the whole selling point behind Acrobat XI - it's trivially simple to copy/paste/extract/edit anything on the page). Password or certificate encryption can also be removed by third-party software if someone really wants to extract your stamp, so the only way to truly secure it is to use digital rights management.