As legal guardian a person is bound by the court to act in the best interest of the protectee. The best option is to contact the GAL or attorney who handled the case. If this is not possible you may write the company explaining the situation and citing the pertinent docket/court designations. Or contact the clerk of probate (or the court of jurisdiction) for further assistance.
If you are referring to unsolicited/pre-approved credit card offers, you can call the national Opt-Out number. This number is maintained by the FTC and will eventually put an end to pre-approved offers for credit. Call 1-888-5 OPTOUT or 1-888-567-8688. You want permanent opt out status.