Stop a System.Timers.Timer which re-enables itself inside its elapsed event handler

I have a Timer that polls an external service. The poll can take a while, so I use a one-shot Timer(System.Timers.Timer) and re-enable it when the operation completes, so I have only one running poll at a time. Now I need to stop this timer externally using the reference and prevent the event from restarting it in a separate thread.

System.Timers.Timer pollTimer = new System.Timers.Timer {
    Enabled = true,
    AutoReset = false,
    Interval = 4 * 1000,
};

pollTimer.Elapsed += new System.Timers.ElapsedEventHandler(
    (object sender, System.Timers.ElapsedEventArgs e) => {

        if (!PollSuccess()) 
        {
            ((System.Timers.Timer) sender).Start();
        } 
        else 
        {
            // Work
        }

    });

// I want the timer to stop permanently here, ie prevent the elapsed 
//event handler from restarting it.

pollTimer.Stop();