When a node is edited I want to always create a new revision. I do not want people to be able to turn this off. How can I do this?
To enabled automatic revision creation go to the content type page (admin/content/node-type) and select edit for whatever content type you wish to have revision created automatically.
Under the workflow fieldset there's going to be an option "Create new revision" make sure you check that box and save your content type settings.
Now under your permissions (admin/user/permissions) be sure to not to give your users the "administer nodes" permission. This will prevent them from overwriting that option.
There's a few different ways to do this, but assuming you want to do it without disabling the
This way, users don't have the ability to change the default value of the checkbox, which is set on the settings page for each content type.