In order to stop the domain mapping one need to perform below mentioned steps:
If registered domain with someone else:
When one had setup the domain mapping, you will have gone to a registrar and changed the DNS settings to ns1.wordpress.com, n2.wordpress.com. To stop mapping that domain to your WordPress blog simply go back and change the DNS. The domain name in Settings > Domains has no effect at all if DNS is not pointing to wordpress blog.
If registered domain with WordPress:
If one has purchased domain from WordPress.com then under the Settings > Domains from blog dashboard, one can find a link to Manage Domains. Click on this link which will be able to receive access to a control panel and allow changing the domains nameservers. Point the nameservers to elsewhere.