Do you think it's possible? If a chemical for example can take off the spray paint and not the original paint underneath? Any idea?

Not likely. If the spray is fresh, acetone might take it off without removing the paint underneath. If the spray paint is cured, you'll have to strip the frame to bare metal to remove it.

I did have success with removing spray paint from my car once. I had backed out of the garage and accidentally ran over a can of spray paint which then shot up on the side of my car. It wasn't a lot, but enough to annoy me. It was like over spray. I used acetone and I did manage to remove it without hurting the factory paint.

However I imagine that the factory car paint was pretty tough compared to the overspray from a leaking can of spray paint that shot up on the side of my car. I didn't remove the paint though for a month after it happened, it wasn't a lot.

If the bike has been completely painted and cured, it will probably be next to impossible to get off without harming the base coat.

It's only the fork, a stupid bike mechanic painted it without asking me. But judging from your sayings it doesn't worth the effort

Try using lighter fluid, spray some on the frame and wipe it off with a rag.
Gasoline will work too. Spray paint will bubble with gas, but not the original paint.
I've also removed over spray with windex and rag

I've used the 99% rubbing alcohol to remove black primer before with success, it also depends on the type of paint that lies beneath. Do a small inconspicuous spot at first and see how it goes.

thinking your origianl paint is enamal- and the new is laquer- use laquer thinner-
Acetone I use quite a bit in making arrows- it evaperates really quick- don't think you'd have much luck dissolving and removing before it's gum

as mentioned- try a small spot first

You can sand it out aswell

I use lacquer thinner, works well.

I had good success using a little bit of Acetone on a clean rag to take some paint off a headbadge and part of the frame. Didn't even have to rub too hard, it came right off no problem and the actually looked like it cleaned up the old paint a little bit.

WTH? complain to his manager and have them replace the fork

stuff to try in inconspicous spots; acetone, mineral spirits, goof off, last resort; ultrafine steel wool

but really, who paints someones bike without permission? they owe you a new one

This "Mechanic" sounds like he needs to stay in his own shop, because he's a tool.

Try some automotive brake fluid, DOT3 or 4. Put a little on a small test area and wait for the paint to wrinkle a bit then immediatly wipe it off and use some cleaner (409, Simple Green, Orange Cleaner, Dishsoap, etc.)to remove any fluid residue. A good quality original finish will stand up much longer to the brake fluid than most rattle can paint so most likely the original paint will not be harmed.

Polishing (or rubbing) compound and lots of elbow grease can take off a thin layer of paint without harming the paint underneath. Depends on how thick the new paint is. And to be clear, I'm speaking of the compounds for automotive finishes, not metal polish.

There has to be Graffiti style removers. I have an old French frame that's been painted and would like to see your results.