this one not work for me.you can block his MAC also like I have done it for my few clients.
/ip firewall filter
chain=forward src-mac-address= your client mac action=drop
http://wiki.mikrotik.com/wiki/How_to_Block_Websites_&_Stop_Downloading_Using_Proxy
but when i followed the instruction all redirected to mikrotik page.The web proxy method is the best way to go.
can u please show me the config.?you can. I make webproxy for 3 networks with one interface of a RB1000.
It could be something with NAT/masquerade.