Do you want to disable it on the server or on the client?
If you want to disable it on the client, just shut down the server and tell NoMachine to never start it at boot up. If you want to disable it on the server, this was intended as a security measure, so that user know that the server is running. Due to big demand, we decided to add the possibility to hide the NoMachine icon until a remote user actually connects. AFAIK this is under development.