I tried to uninstall datamapper using the command gem uninstall dm-core.
gem uninstall dm-core
But it seems that a whole bunch of dependent gems also need to be uninstalled.
gem list | cut -d" " -f1 | xargs gem uninstall -aIx deletes all installed ruby gems!
gem list | cut -d" " -f1 | xargs gem uninstall -aIx