The nightmare loop. Snow Leopard installation

Before I had created a loop with the installation of Snow Leopard, I have the Yosemite and put the dvd in SL. Reset and start installing SL.
As I did not want to continue, prove to exit the installation and select Restart. But the installation of SL began again. And I could not stop it.
I could not get the cd. I tried booting the pc with the C / X..con key, the left mouse button with the command "drutil eject", using Disk Utility SL, but I was saying that files in use and could not. : $
In the end the choice was started with ALT, and could choose to boot DVD, HD and partition recovery.
That nightmare loop.
http://www.applesfera.com/apple/combinaciones-de-arranque-en-mac-os-x
Mac is a Mini 2010.