John,I wonder why it bothers you to have your voice recorded. Would it bother you less if the recording were under your control? If so you can offer to make an official recording of your lectures and have them available. Since the official recording would be of higher quality, this should discourage unauthorized recording. I mention this because the last time I was a student, I was watching the lectures on video and it was very helpful to be able to pause the presentation or rewind a bit on those places where I didn't quite catch what was said.
As to how to prevent the recording, you could possibly flood the room with electrical noise that would disrupt some recording devices. But such a field would also disrupt your students from taking notes on a laptop or tablet so that may not be good (unless you want to discourage electronic note taking). However if someone was determined to make a recording they could get around that by shielding the recording device or using an older recorder that was less susceptible.
Anything you do to make the recording inaudible would likely make your lecture inaudible to your students. One thing you could possibly do is to introduce some distinctive background sound during your lecture that would identify a recording as being taken during the lecture. Or you could get extreme and speak through a voice synthesizer that would make your voice not recognizable as yours.
