You'll need to handle this in your calling code by returning data from your method that will tell your js that it needs to redirect;
[WebMethod]
public static object MyMethod()
{
return new { Success = false, RedirectUrl = VirtualPathUtility.ToAbsolute("~/NotAuthorized.aspx")};
}
Your js would then be something like
$.ajax({
type: "POST",
url: "WebForm1.aspx/MyMethod",
contentType: "application/json",
data: {},
dataType: "json",
success: function (data) {
var r = data.d;
if (r.Success == false){
window.location.href = r.RedirectUrl;
}
}