string source = "去除<a>......</a>字符串<a>......</a>中所有<a>......</a>超链接";
            Regex reg = new Regex(@"<a>\w*\W*</a>");
            source = reg.Replace(source, "");

------解决方案--------------------

C# code

string str = @"<span><a href=""http://www.baidu.com"">baidu</a><font>dfsafsa</font></span><a href=""http://www.baidu.com"">baidu</a>";     
            Regex reg = new Regex(@"<a\s*[^>]*>(.*?)</a>");
            string res = reg.Replace(str, "");//res="<span><font>dfsafsa</font></span>"