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/Peo, FreeWebSpace.net
I don't believe it's possible in WHMCS. You'll need to disable automatic provisioning and manually approve accounts in order to block certain domains from being setup.
I remember seeing this somewhere on the WHMCS site...possibly a mod? I don't remember. It is most definitely going to require some custom coding. In fact, I can't imagine it would be all that hard to do. It's just a matter of doing it.