You can block the access to your site  by using the BrowserCapabilities to detect which browser the user  is running
I made this snippet in c# to detect user browser a while ago....

HttpBrowserCapabilities bc = Request.Browser;
switch (bc.Browser.ToLower())
{
    case "ie": { return ((bc.AOL)? "AOL": "IE" + bc.MajorVersion.ToString());}
    case "netscape": { return "NS" + bc.MajorVersion.ToString();}
    case "aol": { return "AOL";}
    case "opera": { return "OPERA";}
    case "mozilla": { return "MOZILLA";}
    default: return "NOINFO";
}