I would of thought using client certificates would accomplish this.
As for the original issue, this problem extends beyond just VPN and applies to any end client you let on your network that you don't have full control over (think student wifi access and BYOD schemes for example). The tool you are looking for is Network Access Protection and Control. The basic premise is that you have a server on your network that's job it is to interrogate any device coming on your network and ensure it sticks to a policy you set. If it does then the server gives it access, otherwise it gets isolated until such time as the end user conforms to policy.
To implement this you have a several choices, you have easy access to Network Access Policy Server as it comes with Server W2k8. Other than that I've used
Packetfence in the past which is a free opensource solution. With either of these solutions you can carry on using OpenVPN for VPN access