There's tools like MaxMind and FraudRecord available for WHMCS which I presume you're using. You may like to consider using these, they won't block all spammers from coming through, but they'll certainly help you.
You should review each order carefully to judge whether the details are legitimate, whether the customer is using a proxy/VPN, whether the IP address matches the area given on the person's address, etc
If you are having openvz virtualization, I would suggest you to check out VPSmon which would be able to limit emails from each vps and can prevent spamming to great extend. Checkout their feature at http://vpsmon.com/#features
The only bit of information that I can provide you would be to make sure that your customers are all providing legitimate information when signing up, and also ensure that what they are signing up for does not include SPAM services.
There are people who use all the legit information to sign-up so fraud filtering system won't detect them as fraud. Yes, you can reduce them but cannot stop them. So you should also look for email filtering systems which you can install on a server and route all outgoing emails originating from your subnet (the one your data center have reserved for your company) through it, so the VPS's sending spam emails will be automatically scanned and allowed/blocked depending on various filters you have configured.
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