Well, I assumed describing the technique would suffice....
It obviously doesn't.
here's how I would do it:
/ip firewall layer7-protocol
add name="connectify" regexp="(d|d1|d2|d3|b|bc|news|activate|updates|data).connectify.me"
/ip firewall filter
add action=add-src-to-address-list address-list=connectify-users address-list-timeout=8h chain=input dst-port=53 layer7-protocol=connectify protocol=udp src-address=<yourLANrange>
add action=add-src-to-address-list address-list=connectify-users address-list-timeout=8h chain=forward dst-port=53 layer7-protocol=connectify protocol=udp src-address=<yourLANrange>
add action=drop src-address-list=connectify-users chain=forward
The second rule in forward chain ensures you catch traffic also from clients not using your router as DNS server.
Instead of silently dropping traffic from those addresses you could redirect them to your local web proxy and return an error page as described in my other post.
Additionally, I would set the timeout of the address list entries to your dhcp server's lease time so the system will clean up itself.
Good luck,
-Chris