Can I stop buyers from other countries from ordering on my payment form?

There is no setting on the payment form to limit which countries can order.

Votel saves the buyer's billing country automatically, and you can use a country answer to redirect a visitor, but only after they have filled out a form. So put a lead capture form, or any form that asks for the country, before your payment form, and use that answer to send visitors from other countries somewhere else. Without an earlier form there is no way to block them.