How do I stop domain mapping?

If you registered your domain with someone else:

When you setup the domain mapping, you will have gone to a registrar and changed the DNS settings to ns1.wordpress.com, n2.wordpress.com and maybe ns3.wordpress.com

To stop mapping that domain to your WordPress blog, simply go back there and change the DNS again.

The domain name in Settings > Domains has no effect at all if you have changed the DNS.

If you registered your domain with WordPress:

If you purchased the domain through WordPress.com, under Settings > Domains from your blog’s dashboard, you will see a link that says Manage Domains. If you click on this link you will be able to receive access to a control panel that allows you to change your domains nameservers and point it elsewhere. Note that your transfer will not be complete until 60 days after you registered your domain at WordPress, in accordance with ICANN.