disconnect client from server side signalr

You cannot stop and start SignalR connections from the server. You will need to call

$.connection.hub.stop(); //on the client before the user attempts to log on and then call 
$.connection.hub.start(); //after the log on attempt has completed.

One way you could do what you ask is to write a disconnect event on your client that the server can call through SignalR. Maybe something somewhat like this:

myHub.client.serverOrderedDisconnect = function (value) {
    $.connection.hub.stop();
};

Then, on the server, something like this:

Clients.Client(Context.ConnectionId).serverOrderedDisconnect();