string str=@"爱爱<b>ssdfsddf<.d></b>天天
爱爱<span skid></span>天天
";
str=Regex.Replace(str，@"(?is)<(span|b)\s*[^>]*?>.*?</\1>
","");

------解决方案--------------------

Regex.Replace(str, @"(?is)<(span|b)[^>]*>(.+?)</(span|b)>", "");