How to permanently remove URL from Google

The removal tool only provides temporary removal for approximately six months. To permanently remove content or URLs from Google search results, make sure your web server returns a 404 (Not Found) or 410 (Removed) HTTP status code.

protected void Page_Load(object sender, EventArgs e)
{
    Response.StatusCode = 410;
    Response.Status = "410 Gone";
}

Refer : https://support.google.com/webmasters/answer/9689846#make_permanent