Use the gemset empty command:
gemset empty
rvm gemset empty mygems
This command removes all the ruby gems installed locally in 1-stepWorks well in Ubuntu 10.10
gem list | cut -d" " -f1 | xargs gem uninstall -aIx
PS - removes all local gems. Use sudo accordingly.
rvm gemset empty <gemset name>
This will remove all gems from your mentioned gemset.