So when i build a client.exe it can listen using listen -game command and other clients can connect to it, i want people to only be able to connect to the dedicated server and remove the listen functionality from client code, is that possible ?
Having the same problem , the only solution i found was to use this solution to separate client and server code : [link text][1] [1]: Separate Server and Client code during builds. Possible in blueprints? - Blueprint - Epic Developer Community Forums
This will not prevent the clients from creating listening servers but it will make their listen servers useless 
Hope this helps .