This is something we need very often. Either for security reasons or just because of some business rules. I will share a small code snippet with you as an extension method to string type so you can take advantages of using Regular Expressions to remove HTML tags from any user input.

public static string StripHtml(this string value)
{
    if (string.IsNullOrEmpty(value))
    {
        return string.Empty;
    }

    value = Regex.Replace(value, @"<(.|\n)*?>", string.Empty, RegexOptions.Multiline | RegexOptions.IgnoreCase);

    return value;
}

And this is how you need to use it:

string var = txUserInput.Text;
var = var.StringHtml();