I found a solution that does not use the "auto-submit" feature at all, but still does the same effect (Automatic Authentication)
I have the MAC addresses of the LAN computers. So what I did was to put them all in the "MAC Pass-through" section of the Captive Portal. This assures that "subscribed" clients will automatically have Internet access without any annoying, periodical login screens.
But what about the "unsubscribed" (AKA blood sucker, AKA leecher) clients? Simple!
They get the screen that I've attached to this reply!
And here is the mega-hard-coding! (No forms!)
<html>
<head><title> Wi-Fi Network </title></head>
<body>
<br>
<div align="center">
<img src="wifilogo.png" />
</div>
</body>
</html>
Basically, it says "Computer not registered" and informs the person about my phone and apartment numbers.
So, unsubscribed and unwanted guys stay out, friends get in automatically.
Isn't that great?