I read through the user manual for your router from Asus' website, and it doesn't look like you have the ability to block specific hosts.
You have two options (maybe more, but these are the easiest, in my opinion):
Custom Firmware
You can install a custom firmware that adds the functionality of blocking specific traffic from a website. According to the DD-WRT router database, your device is supported, so you could try using their firmware
Be warned, you do run the risk of bricking the router if any process of installing custom firmware goes wrong. If you follow their instructions closely though, you should have any problems
DNS Filtering
I personally use OpenDNS Free edition for my small office. After setting up an account, you can set your router to use their DNS server, and block specific domains in the dashboard. All computers that are set to use the DNS server that the router is providing will be blocked.
Replace the Router
Many, many routers support blocking hosts. You may have just picked one that doesn't. The easiest option would be to simply replace the router with one that can do what you want.