If you no longer need an add-on domain in cPanel, you can remove it easily to free up space and keep your account organized.
1. Log in to your cPanel account.
2. In the Domains section, click on Domains.
3. Locate the addon domain you want to remove and click Manage next to it. 
4. Scroll down, click on Remove Domain, and confirm the removal.
You should see a message that your addon domain has been removed.