503: Too many IP addresses accessing one

My site is having the same identical issue as outlined in the post listed below.

http://www.s2member.com/forums/topic/access-issue/#post-23999

I created a IP Restriction page as well and and created the file in the post 23999, (added the s2hacks.php in the

wp-content/mu-plugins/s2hacks.php

directory, which I had to create as it didn’t exist.

So when I finished and (of course changed the URL in the code to my page) I am getting this error at the top of my page

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/tbpwnet/public_html/wp-content/mu-plugins/s2hacks.php:1) in /home/tbpwnet/public_html/wp-content/plugins/event-espresso-free/espresso.php on line 54

I have since removed the code and the Warning has gone away. Not a programmer, but I can follow directions, just wondering what I missed?

Any help would be greatly appreciated.