PPTP, OpenVPN, IPSec?
There's not an easy way of doing it unfortunately. OpenVPN has no built-in method of blocking client IP addresses AFAIK. The most obvious place to do it would be on the firewall. But because of the way the router dynamically creates the firewall rules it's difficult to implement reliably.
There was a similar discussion here, but that was for a blacklist rather than a whitelist. I think the take-away from that thread was that instead of relying on IP addresses it would be much more preferable to use a password+certificate combination.
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