Preventing google search for custom top-level domain
To just avoid connecting to Google, open Safari's preferences, click on "Search" and then change the search engine to something else.

To prevent that domain from going to any search engine, add a Safari bookmark with the full URL (including the http:// or https:// protocol). Once the domain is bookmarked it should come up when the user begins typing the name in the address bar.

The user's machine will still need a way to route traffic to the custom domain, either via DNS settings, or by adding an entry to the /etc/hosts file on the user's machine.