Using Guardian to block a top level domain
When adding domains to a block category, you can just add "ru" or "uk" for example. This would effectively block access to all domains using this top level domain (TLD).
Blocking access to an entire country is another matter because material originating in a country can be easily served from other domains, hosted in other countries as well.
Geo-location IP addresses are not very reliable either and the same issue applies to IP address blocking.
While both methods do have an effect, it's not a method that can be trusted to reliably block access to or from a specific country. Using HTTPS inspection, blocking access to sites with invalid certificates and anti-malware scanning should all be used to get the best protection from malicious sources.