How to prevent sending of hostname via DHCP?
I was trying to answer the opposite question: why does my Mac NOT send a hostname in DHCP requests. 

After discovering that some Macs send hostnames and others do not, I found the cause in my case: spaces in the Computer Name.

For the gory details, see my Server Fault posting. if you don't want your computer to send a hostname, you could put a space or another unusual character in the Computer Name, and that should do it.