I thought I was going mad (maybe I still am :cheesy: )
I spent hours disabling modules on my test 1.7.0 site after changing the template to afterburner, but still mootools would load. I ended up with only the menu module loaded, and almost every site plugin disabled as well - but still it loaded.
So I downloaded Joomla 1.7.2 from the joomla website, installed it with the sample data, checked that it loaded mootools using firebug net. I then installed afterburner, and setting captions enabled to no works fine, but setting mootools enabled to no isn't working at all -it still loads mootools-core.js