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Almost impossible task. If you can, activate Invite only registration in JomSocial so only existing registered users can send invites to new users, but even that is not 100% guarantee that there will not be unwanted registrations.
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it does not seem unwanted. Captcha is used to protect from spam.
If you want to block users to register on site you can approve the user yourself,
From user > options > User activation >admin. You cant restrict a specific user to register on-site except you blocked country to access your site.