Cannot register my second-hand unit *ID already used*

Hello, I have a second-hand (previous owned) tracker and I can´t register it in my account, wialon give me this message:

"(phone number, name or UID) already exists"

The previous owner had it registered in orange.gps-trace.com and now he won´t answer my messages to remove the ID.

Is there a way to remove the UID from his account so I can add it to mine ?

thanks