If you no longer need them, you can delete databases from your cPanel account.

Warning: Make sure you back up your database before you delete it.

From your client area go to:

Services -> My Services -> click on the green "Active" button -> Login to cPanel/Control Panel

In the account Dashboard, click cPanel Admin.

In the cPanel Home page, in the Databases section, click MySQL Databases.

In the Current Databases section, in the Actions column of the database you want to delete, click Delete.

Click Delete Database.

More info:

For help, see the cPanel documentation on the MySQL Databases feature.