When you install opencart all countries are enabled.
If you want to disable them in one step, you need to:
- Go to your PHPMyAdmin
- Click the SQL tab
- On the left side under 'Recent tables' there should be an entry 'oc_country' or something like that, depending on what was entered during the installation process
- On the right side in the field under 'Run SQL query/queries on database [databasename]', enter the following command:
'UPDATE oc_country SET status=0' (replace 'oc_country' with the appropriate name; see #3)
- Click the 'GO' button on the bottom-right
- All countries should be disabled.
- Enable the countries you need.