For terminating BGP sessions, delete the associated resource, such as the Cloud VPN tunnel or VLAN attachment. After you delete the resource, the session is automatically terminated. You can terminate BGP sessions manually by using the gcloud command line tool. If you do, the tunnel or VLAN attachment remain, but Cloud Router doesn't advertise routes over them.
remove-bgp-peer command, as shown in the following example:
gcloud compute routers remove-bgp-peer my-router --peer-name my-bgp-peer