Gateway already used, Change EUI

Hello, we’re stuck on a problem, another account is using our gateway.
We can’t contact them, so they can’t delete it for us. Actually we are beginners in Lora, gateways and TTN.
Do you think it can be possible to change our Gateway EUI, or something that can help us?
Btw our gateway is the Kerlink iFemtoCell and his EUI is '“7276FF003904061B”.
Thanks

Is this a new gateway? Of did you buy it from a non reseller?