Use phpmyadmin to access your Tiki database and browse the "tiki_preferences" table. The "name" column contains the names of the Customization options, some of the names are:
The header_custom_js is probably the one you are looking for. If there is a row with that value in the name column, edit that row and remove the code you added.
I tried to help someone else with a very similar problem in an earlier thread. Some of the information in that thread might be of interest to you: http://tiki.org/tiki-view_forum_thread.php?comments_parentId=44013
I also contacted him directly and offered to help, but he decided to move his site to ning, so we never did find a solution.
I hope you have better luck.