So, does a fly have ears?

Hmmm... http://www.acfnewsource.org/science/fly_mic.html

Looks like Eceeng's device might scare off Cornell University scientists as well...

 

well that begs the question what does unpleasant mean ,
do they even have hearing abilities?

One sure thing is, any loud noise is uncomfortable and we always trying our hardest to avoid it. So, does a fly have ears?

No, a fly doesn't have ears, but has tiny hairs that would vibrate at such frequencies.
Also, since I can hear a fly flying (zzzzzzzzz), I assume that the ressonant oscillation for the wings is not ultrasonic. Just like a pendulum, the fly wings have their optimal oscillation, and thus minimal loss of energy occurs when the fly is flying using the natural oscillation of the wings. So, the fly wings would not visibly vibrate at ultrasonic frequencies.

 

You could design and build a teleporter which accidentally combines the fly with yourself at molecular level. Then market it as a fly repellant. But you'll only have a short amount of time to get it to market once you've tested it....