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