Sorry m8, but it doesn't appear as though the D-Link DI-704 router has a "filter" function to block specific IP addresses from the WAN (internet) side. My SMC router is uncannily similar in terms of the web admin interface, so you might get lucky and be able to find a newly released version of a flash upgrade for your router. The last flash ugrade for the SMC included this "filter" function.
But... there is hope..! I've reached the limits of my router, so I'm playing with Freesco now. It a router/firewall on a floppy disk, Linux of course
You simply have to download the floppy disk image, pop the disk into an old 486 fitted with two network cards, boot, and you have a very powerful router! Then it's easy to block specific IPs
The next step is a "reactive" firewall, which is capable of detecting portscans/DoS attacks etc. and then it implements firewall rules automatically. Sweet eh?
There should be a whole major new release of Freesco out very soon now, but the current release is more than enough for the most demanding users. Did you know that you can become an ISP if you fit a modem to the Freesco router? LOL!
http://www.freesco.org/