n'Abend Michael,
> What's the simplest and hopefully efficient way to block domains from being
> resolved by pdns-recursor?
>
> I'd like to just NXDOMAIN being returned for all RRs in unwanted domains.
I think your best bet is using the Lua interface [1].
Regards,
-JP
[1] http://wiki.powerdns.com/trac/wiki/Lua