Sorry, 10.0.11.38 has been banned For my website when using Firefox or Chrome you get the following message:
Sorry, 10.0.11.38 has been banned.
For Internet Explorer you 403 forbidden error.

Note: 10.0.11.38 is the internal IP of the load balancer which has the domains certificate and associated public IP address.
I went to admin/user/rules and noticed that this ip of 10.0.11.38 has been listed as a host to deny access so I deleted this rule and the drupal website displays again.
We did not enter this ip as a rule originally so I suspect that drupal somehow decided to add this ip as a denied host.

How would I prevent this from happening again?
admin/user/rules is not a path in a regular Drupal install, so you must have some module that does this for you. Since you tagged your post with domain-access I can only guess that the one. You should start looking into the settings of whatever module holds that path.