We see a number of questions by users in the Blogger Support Forums about how do they remove a Custom Domain that they had setup for their blog.
It's very easy to switch back to your old blogspot address.
If you are still using the Old Interface, log onto your blog and go to the Settings, Publishing page.
Then click on the Switch to blogspot.com link.
In the New Interface you go to Settings, Basic and click the "X" next to the domain name to remove it.