Their method of using voice recordings as evidence of a landing is fundamentally flawed for many reasons.
For parking, regos are recorded by hand by the ARO or similar and passed on.
I have found that if I query an invoice for landing fees collected by radio, they almost immediately remove it.
If I query a charge for parking, they refer back to the ARO / reporter of the info.
If I were you and you firmly believe that someone is using your rego, Avdata should be able to retrieve the recordings which may help you/them identify the perpetrator and subsequently end up as a Police charge of OPBD or similar.