My cats occasionally try to mooch human food from me. For instance, if I'm
eating a TV dinner in the living room, they will approach, sniff, and
occasionally try to snag some tasty morsel from me.

If I tell them to keep away in my "no!" voice, it doesn't have much effect.
However, I've found that if I hiss at them, as much like a cat as I can
manage, Bebop will stop immediately. Samba is not so easily deterred by the
hiss but can be dissuaded by gently pushing her away.

I'm just curious whether anyone else has ever tried hissing at their cats to
make them stop doing something and what, if any results, you got?

I'm pretty much convinced that a hiss from one cat to another means "Back
off!" - no more, no less - and it works very well on Bebop. If he's bugging
me while I'm trying to sleep, hissing works very well on him. Unfortunately,
Samba is not quite as attentive to hisses - at least not mine! Then again,
she's also pretty feisty and isn't shy about ignoring Bebop's hisses when
she wants to. Maybe she's just employing a little selective deafness with me
too :-)