I run each signup through maxmind, review the domain names and have a tech check the server periodically also. this method has worked well for me for about 5 years now.
We started using MaxMind about 6 months ago. Since then we're received ZERO chargebacks. This is seriously a good investment since the merchant account providers charge fees for chargebacks. A minute fraction of that can go to a low cost MaxMind subscription.
I have used maxmind for some time, have to be careful getting the settings right though too low and you are blocking good signups too high and its not doing its job.