I was looking for a way to cancel a ReactiveCommand<T> in ReactiveUI but could not find any good official documentation on this. Instead I came up on Gluck's solution on StackOverflow (surprise!). This is one way you can cancel a command using ReactiveUI 6:

public class ThisViewModelCanCancelCommands
{
    public ReactiveCommand<Unit> Poll { get; private set; }
    public ReactiveCommand<object> Cancel { get; private set; }
    private IPoller poller = new Poller(); // Dummy service for example;

    public ThisViewModelCanCancelCommands()
    {
        Poll = ReactiveCommand.CreateAsyncObservable<Unit>(
                this.WhenAnyObservable(vm => vm.Poll.IsExecuting).Select(v => v == false), // In order to disable button when executing
                _ => Observable.StartAsync(PollHandler).TakeUntil(Cancel)
                );

        Cancel = this.WhenAnyObservable(vm => vm.Poll.IsExecuting).ToCommand();
    }

    private Task PollHandler(CancellationToken ct)
    {
        return poller.RunAsync(ct);
    }
}