score:9
After a family member lost and found a passport, I asked a CBSA officer at the Ottawa airport enrollment centre if Nexus cards should be treated the same (i.e. mailing it to Passport Protection). They said "treat it like a credit card; a couple snips should do."
While it's not official written/documented advice, it's good enough for me. A single small cut through the antenna is good enough to render the card functionally useless -- a few more for privacy (especially through the data area on the back) makes sense.