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Not sure if your psn account info is stored on the card, but anyways just go into your account and change your password all your credit card info is on sony's servers. If you change your password they wont be able to get into the account. You should change your password a few times a month anyways just to be safe.
I think I would start with changing my password. I don't know for sure, but it may make it so he/she has to resign in under the profile to access your stuff. At the very least I would think it would make it so he/she couldn't access the PSN Store without having to resign in.
Link. Hope this helps. There's also a support number: 1-800-345-SONY (7669)