[Update]
Since Docker 1.13.x you can use Docker container prune:
docker container prune
How to remove old Docker containers - Stack Overflow
docker prune commands
$ docker container prune
$ docker system prune
Reference
When you run docker ps -a, you might have too many containers and want to remove old containers. you can remove those containers as below:
$ docker rm `docker ps -aq`
If specified container is running, you’ll get an error:
Failed to remove container (f6259edb8236): Error response from daemon: Conflict, You cannot remove a running container. Stop the container before attempting removal or use -f