Unfortunately not all browsers respect the expires directive, you might try the following to add the no cache pragma to the headers when you output the login page:

tachyon

# if using CGI.pm to generate output try:

print $query->header( -type          => 'text/html',
                      -expires       => '-1d',
                      -Pragma        => 'no-cache',
                      -Cache-control => 'no-cache');

# if directly outputing try:

print <<END_OF_HEADER;
Status: 200 OK
Expires: now
Pragma: no-cache
Cache-control: no-cache
Content-type: text/html

END_OF_HEADER
[download]

This should generate a page expired message when you try to go back.