the following is what I received from CIC.
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Sir, Madam,
Thank you for contacting Citizenship and Immigration Canada. I am pleased to follow up on your request:
Voluntary Relinquishment of Permanent Resident Status
Permanent residents who no longer wish to have that status may advise Immigration Officers overseas or at a Canadian port of entry.
Provisions exist for clients to voluntarily relinquish their status in exceptional circumstances.
A person may be allowed to relinquish their permanent resident status if an officer determines that the individual is willingly giving up their status and fully understands the consequences of that choice. If the officer is satisfied the client is asked to sign a declaration to that effect and can then be considered as a foreign national.