How do I come up with a comeback for this?

Fuck Off works for most things.

Heee hee hee hee. “Back”... “Bone”...

“At least mine’s not stuck up my ass!” (That’s a little vicious, not playful).

Was it a man or a woman who said it?

If a man, how about “lend me some of your viagra and it’ll be stiff enough”.

If a woman, “get me some of your man’s viagra and I’ll have bone enough.”

O.k. I’ll take yours.

Tell him/her to ask your fluther friends but jellies don’t have back bones. Thanks for pointing that out. ;)

“Did you think that up yourself or do you have a stable of dialog writers?”

I think @johnpowell nailed it.

I grew a bone, in your mother’s back.

…& what if i’m comfortable being an invertebrate, eh…cunt?

You take 2 fingers to imitate a Hitler Mustache and then use the other hand to do a Nazi salute right into his eyes and then you say to him “Grow a new eye.”

How about the following:
I will as soon as YOU get off my back!