The countries field on a BBIS form displays all active countries listed in CRM > Administration > Countries and states.

To add additional countries, they will need to be added in CRM.

To remove countries from the Country field on BBIS forms there are two options:
1) Mark the unwanted countries inactive in BBCRM.
2) Add custom JavaScript to the page to prevent the unwanted countries from showing up in that area.

Additionally, there is an Idea Portal suggestion to add the ability to remove countries from BBIS forms without marking them inactive: