Hi Domini Beniston,
If you do not want a 3rd party to make a claim/act on your behalf then you should not sign authority for them to act on your behalf or nominate them to receive a repayment on your behalf.
If you authorised an agent to act on your behalf in the past and you no longer want this to be the case, then you would need to contact HMRC to update your records.
If you signed a deed of assignment this can only be revoked by both parties writing to HMRC revoking the deed.
If you think you have been the victim of a scam claim then you would need to report the details to:
Action Fraud 0300 123 2040 website www.actionfraud.police.uk