Zoiper receives the calls that are sent by the server.
There is no way to stop the calls from inside zoiper. Is your computer in an internal nettwork, or it is using a public ip ?
You could try to block port 5060 except for the ip of your Vo Ip server. You might also try changing the Zoiper sip port in the Advanced settings to something else than 5060. The server will not have any issues with this as you are registering to it.