I have an Asus RT-N66u router at my workplace. How do I block an employee from using the desktop app to stream music?

block the IP.

if your router doesn't do it, just add it to the HOSTS file on his PC.

As Dave says, this is really a Human Resource problem.

If the employee has only a user account on the PC, you can add entries in the host file on the PC such as:

answers.yahoo.com 127.0.0.1

Replace the URL with whatever URL you want to block. 127.0.0.1 simply points any DNS look-up back to the PC itself so although the DNS look-up works, but the real site is not available. You can add as many entries as you like. Each entry goes on a new line and each points to the same IP address. The hosts file takes precedence over the DNS server.

The next level would be to introduce a PC based firewall that logs all accesses, and blocks access to forbidden sites.

I hope this helps.

Honestly, this is an HR problem, not a technology problem. Have HR release a policy to the employees detailing what is and isn't allowed. Let the employees know that traffic will be monitored periodically and what the repercussions are if caught.

If you can kill access to IP's, kill these:

78.31.8.0/22

193.182.8.0/21

193.235.232.0/24