Well I would try mechanically first, gently with your finger nail, ASAP off the screen. Worry about the casing later because it's aluminum and simpler. You are kind of in a bind for the screen because you must apply a solvent to remove the spray paint, but solvents will also dissolve the coating on the surface. First you need to identify which MBA screen you have (i.e. coated with silicon dioxide and niobium pentoxide, or magnesium fluoride and polymer). Then you need to identify the type of spray paint (i.e. oil, latex).
Take a look at this link for solvent types for removing spray paint:
http://city.milwaukee.gov/AntiGraffiti/Graffiti-Removal-Techniques.htm#.Vrmr98dTLH8
The damage to your screen coating can be extreme if you use the wrong solvent. Look at this guy that used an 'alcohol swab', which was likely impure isopropanol...coating ruined.
https://discussions.apple.com/thread/5855837?tstart=0
Maybe his concentration was too high or it had contaminants, who knows he doesn't provide info. Some people clean monitors (no coating) with diluted isopropanol and have no issues...Personally I have used diluted ethanol (laboratory grade) for years for cleaning, but I don't have an anti-reflective screen coating and the purity of my material is the highest possible. Honestly I think it's acid contamination in low-grade alcohol that is the culprit, rather than simply the alcohol because these low molecular weight alcohols are so volatile (quickly evaporate).
There is conflicting opinion on what actually removes the coating...
http://www.siliconexit.com/2015/07/...laim-that-improper-cleaning-causes-staingate/
Some people believe it is "oil secretions from fingers", which is rubbed off the keys onto the screen when the lid is closed. These sweaty oils are slightly acidic, which would attack the oxides and fluorides with free hydrogen ions.
Get back with detailed information on the screen and paint, and maybe one of those suggested solvents would be suitable for your chemistry if you have a coating... but realize that it is very, very risky applying any solvent to electronics/plastics, especially a surface featuring a thin coating. Removing spray paint from a coated screen is tricky and requires careful forethought.
You are probably better off just trying extra virgin olive oil, TBH, if it's the right type of paint. You can try that on the aluminum paint chips first. If you have moderate success then try olive oil and rubbing with a lens cleaner/microfiber cloth, before attempting a solvent approach if you have a coating. I've never personally put oil on a screen, and I expect it will be tough to get off. But oil then soap then damp microcloth should work in theory (oil -> amphipathic -> water). Some people swear by oil-based cleaning techniques for smudges on computer screens... High quality extra virgin olive oil would not have a pH (acid/base) because there are no free hydrogen ions. As oil ages the 'acidity of oil' increases because its lipids oxidize, but EVOO is more resistant to this than other oils. Regardless, the pH drop in aged EVOO is trivial compared 'sweaty oil secretions' from someone's hands. It's probably pH ~6.5 for heavily aged and rancid EVOO (I don't have the patience to calculate it exactly) whereas sweaty oil is pH ~4.5.
Anything you try, do a tiny patch of the bottom left corner of the screen first, then wait till the next day to observe for any damage to the coating. BTW I am assuming this is not covered under Apple Care, or they have no suitable/recommended solution for you.
