I know that we can ban IPs using the "Kill User Activity" admin applet, but is there a way to close our forum from all visitors from a specific country?

Theoretically, you have all for this. For instance, when we open a user's card (such as /Forum/default.aspx?g=profile&u=423), we see the country of the user. I.e. you can detect it, and you already have the mechanism that blocks users by specific IPs.