How can I completely disable the sound on an iMac?

I work at a high school where I am in charge of all technology on campus, as well as the position of librarian (an effort to save money by the disctrict). In the library, I have about 14 iMacs (2008) that are available for students to do research and reports on, though the computers have basically been turned into really expensive Facebook and YouTube machines. Anyways, I'm looking for a way to completely disable the audio on these iMacs. Simply muting the volume and hiding the control on the toolbar doesn't help because there's volume control right there on the keyboard. Is there any way to just disable the sound all together? Such as disabling the sound card or possibly usuing a utility or script? An even better solution would be to disable the speakers, but leaving the headphone jack enabled so if need be, a student can plug in headphones and still get sound. But I'll settle for completely disabling the audio, if anyone can help with that. Thanks so much!