Worke5r86 how you are imagine this?
Like.. you see a checkboxes with countries and check all unneeded except one and click Save? 🙂
If you need to limit registrations you can use invites or just some referalls system. Also you can try to prevent registration based on national email-domains of your country.
All your requests about country limits based on GEO will depend on IPs databases which must be fresh and all time updated (it's a paid service, e.g. MaxMind, etc). Of course, Flarum have some solution for restrict register/login by IP, but it's unreal to use it like firewall. This databases used for restrict access via IPs, so it's more related to WAF/AntiDDOS solutions which operate in front of Flarum. Also, this can be done with Cloudflare WAF.
Good luck 🙂
