Reinstall Mac OS X Snow Leopard on MacBook Pro, after installing El Capitan

I have an old MacBook Pro and wish to reinstall the original Mac OS X Snow Leopard on it, from the original install DVD, that I have. I've previously been using El Capitan on it and can'd find any way to go back to Snow Leopard...

I restart the MacBook holding Command and R, which takes me into the Utilities - from here I can reinstall El Capitan (which I don't want to do), erase the disk etc., when I go to select the boot drive, I'm given the option to select the Snow Leopard Installation DVD, which I do. When the MacBook restarts, it goes to the prohibitory screen ( I presume as I'm trying to go back to an older operating system?). Is it possible to remove all traces of El Capital (the disk image/base system, as I assume this is what's stopping me going back to Snow Leopard)?

Thanks. Ben.