You can use a simple regex like this:
public static string StripHTML(string input)
{
return Regex.Replace(input, "<.*?>", String.Empty);
}
Be aware that this solution has its own flaw. See Remove HTML tags in String for more information (especially the comments of @mehaase)
Another solution would be to use the HTML Agility Pack.
You can find an example using the library here: HTML agility pack - removing unwanted tags without removing content?