I've tried searching the php manual and internet on how to delete cookies and I've tried it the exact same way they all say:
setcookie("name", '', 1);
setcookie("name", '', time()-3600);
setcookie("name", $value, time() + 259200, $path);
Cookies must be deleted with the same parameters as they were set with. If the value argument is an empty string, or
FALSE, and all other arguments match a previous call to setcookie, then the cookie with the specified name will be deleted from the remote client. This is internally achieved by setting value to 'deleted' and expiration time to one year in past.
So also make sure that
$path is specified correctly -- also when deleting it. For instance, if the cookie was specified in a subdirectory, you may not be able to delete it from either the parent or children directories (or both).
I'm not entirely sure how the permissions work, but you might want to use the Web Developer Toolbar to view what the path is of the cookie you are attempting to delete.