How do I disable the Heading 1 format?

Note that if you stop now you'll experience a slew of JavaScript errors. To avoid these, you'll need to remove the offending JavaScript alert from the following files:

root/editor/js/fckeditorcode_gecko.js:
root/editor/js/fckeditorcode_ie.js:

else alert("The FCKConfig.CoreStyles['"+E+"'] setting was not found. Please check the fckconfig.js file");

This will take care of the JavaScript error and your Font Format listing should load as intended.