Omeka (as you've all noticed) doesn't have any remotely convenient way to remove Elements from the admin form for Items.
This is probably a perfect use case for a plugin, but Omeka doesn't quite currently expose the kind of functionality a plugin would need to be able to do this.
This request really hasn't gotten any action since Jeremy's post a year ago, but I think it's a feature that would make sense for Omeka 2.0 (the next major version), whether as a plugin or core feature.
In the realm of temporary workarounds, it probably wouldn't be too difficult to come up with something in JavaScript or CSS that removes the unwanted form elements from the page. It's not the ideal solution, but it's something somebody could work on without waiting for a new Omeka version.