There is no more com.apple.mDNSResponder.plist in OS X 10.10. The service that controls mDNSResponder is now discoverd. The plist file you are looking for is actually:
/System/Library/LaunchDaemons/com.apple.discoveryd.plist
You want to add --no-multicast
to the ProgramArguments in that file.
<key>ProgramArguments</key>
<array>
<string>/usr/libexec/discoveryd</string>
<string>--udsocket</string>
<string>standard</string>
<string>--loglevel</string>
<string>Basic</string>
<string>--logclass</string>
<string>Everything</string>
<string>--logto</string>
<string>asl</string>
</array>
Becomes
<key>ProgramArguments</key>
<array>
<string>/usr/libexec/discoveryd</string>
<string>--udsocket</string>
<string>standard</string>
<string>--loglevel</string>
<string>Basic</string>
<string>--logclass</string>
<string>Everything</string>
<string>--logto</string>
<string>asl</string>
<string>--no-multicast</string>
</array>
You'll want to be very careful when editing that file. You can also use this script to do it for you - https://raw.githubusercontent.com/MacMiniVault/Mac-Scripts/master/disablebonjour/disablebonjour.sh