I can do a killer Forest impression but it's really low in volume so it wouldn't work, plus I don't think they deserve kindness. Usually I find they ARE joking but it's you that's meant as the butt of their joke. If you notice whenever someone yells something out they are never alone.
Also I used to get this a hell of a lot more when I was skinnier. I would still flip the bird or yell f*** you though.
Now if I actually heard it I'd run right up to whoever it was and stop right dead in front of their face, and ask what did you say...with clenched fists. That usually shuts them the f*** up. And hopefully they'll think twice about harassing another runner again.