How to completely disable Airplay in Mac OS X
I have just found some help here: Disable AirPlay Mirroring. It will let you disable Airplay Mirroring (I assume that includes just the sound) by using a terminal command. It will deny the computer access to the AirPlay.app that allows Airplay mirroring.

sudo chmod 000 /System/Library/CoreServices/AirPlayUIAgent.app/Contents/MacOS/AirPlayUIAgent

Some problems are associated with this though. I've copied the article's "drawbacks" below.


  The only drawback to this method is that it locks up the menubar, which means your users won't be able to use any of the menu bar items. They're still able to adjust the volume using the keyboard, though. Logging out and back in will restore the functionality of the menubar until the user tries to use AirPlay mirroring again. I guess if your users complain about the menubar crashing, you know what they've been trying to do! To remove your users temptation of crashing the menu bar all the time, you can hide the AirPlay icon by opening System Preferences > Displays and unchecking the "Show mirroring options in the menu bar when available" checkbox. 


if you don't enjoy it afterwards and you want to fix it back to defaults without reinstalling OS X you want to do the same steps above like you did when disabling AirPlay. But instead of the 000 after sumo chmod you want to use the numbers 755

You would follow the steps, restart your iMac like before and you should have re-enabled it back