Came up against this recently when trying to upgrade a Rails app which was using MongoDB. Obviously I didn't want to use ActiveRecord, so I set about removing it.
The old way of removing ActiveRecord was to place this line in the environment.rb file, "config.frameworks -= [ :active_record ]". However, in Rails 3 you can simply pass the "-O" switch to the rails command, like so:-
rails yourapp -O
But what if you've already created your app? If you look in your config/application.rb file, you'll see this line at the top:-
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