How do I stop domain mapping?

This applies when you did NOT buy the domain at wordpress.com

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

Go back there and change the DNS again.

We do not have control over your blog.
We do not stop you changing the DNS

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