You can choose to limit a voucher to specific countries only.
If you choose to do so, the voucher will be valid only for users whose IP address is in one of the countries selected.
To limit vouchers:
1. Go to Admin/Voucher and find the voucher you want to block.
2. Scroll to find the 'Countries' header and choose the country (or countries) you want the voucher to be valid in:
3. Click 'Update' to save the changes.