How to get filter to redirect to another action?

RedirectToAction is just a helper method to construct a RedirectToRouteResult(), so what you do is simply create a new RedirectToRouteResult() passing along a RouteValueDictionary() with values for your action.

Complete sample based on code from @Domenic in the comment below:

public class IsGuestAttribute: ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if (!Ctx.User.IsGuest) 
        {
            filterContext.Result = new RedirectToRouteResult(
                new RouteValueDictionary 
                { 
                    { "controller", "Home" }, 
                    { "action", "Index" } 
                });
        }
    }
}