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.
As far as I know you're correct, there is not an easy way built-in to the gem command to do this.
However, you can check out gem-prune which can help clean up your gem repository after you've removed dm-core.
http://github.com/ddollar/gem-prune/tree/master