Can I Unsubscribe from a Yearly/Monthly VPN Gateway?

Yes.

On the VPN Gateways page, locate the row that contains the VPN gateway you want to unsubscribe and choose More > Delete in the Operation column. After you unsubscribe from a yearly/monthly VPN gateway, all VPN connections created for the gateway will also be deleted and cannot be recovered.

After the unsubscription, the remaining prepaid fees will be refunded.