3
           
            
                
     
    
    It is not difficult hack, I think.
But this code is not tested.
line 42 in include/checklogin.php
 $member_handler =& xoops_gethandler('member'); 
$myts =& MyTextsanitizer::getInstance(); 
$user =& $member_handler->loginUser(addslashes($myts->stripSlashesGPC($uname)), addslashes($myts->stripSlashesGPC($pass))); 
[color=ff0000]// start of hack for preventing from multiple logins by GIJ 
if( is_object( $user ) ) { 
    $online_handler =& xoops_gethandler('online'); 
    $online_handler->gc( 300 ) ; 
    $onlines =& $online_handler->getAll(); 
    foreach( $onlines as $online ) { 
p        if( $online['online_uid'] == $user->uid() ) { 
            $user = false ; 
        } 
    } 
    if( is_object( $user ) ) { 
        $online_handler->write($user->uid(), $user->uname(), time(), 0, $HTTP_SERVER_VARS['REMOTE_ADDR']); 
    } 
} 
// end of hack by GIJ [/color] 
if (false != $user) { 
    if (0 == $user->getVar('level')) { 
        redirect_header(XOOPS_URL.'/index.php', 5, _US_NOACTTPADM); 
        exit(); 
    }  
This hack doesn't work fine with auto login hack.
And don't forget logout.
If you leave the site by closing browser, You can't login 300 sec.