I'm rather disappointed to find almost no useful information on how to remove a specific painting substance from a specific type of painted wall. That's surprising since graffiti is a widespread problem and I guess millions of people face it every year. The typical answer indeed seems to be "repaint and get over it" yet repainting would be a lot of hassle in my scenario - even sub-optimally removed graffiti would be beneficial.
So I'll describe my findings here. Fact is one has to find an appropriate chemical which is usually some organic solvent like acetone. Each solvent will have two key parameters. Parameter one is whether it dissolves the painting. Parameter two is whether it dissolves the original paint. Obviously you're only interested in solvents that do dissolve the painting and don't dissolve the original paint too much and the don't dissolve the original paint more that they dissolve the painting.
So I was combatting spray paint and permanent marker used over some kind of oil-based or maybe nitro original paint.
The "white spirit" solvent would dissolve the spray paint very well and wouldn't dissolve the original paint at all. So I wasted something like a half roll of paper towels to have about one square meter of surface cleaned and the spray paint was removed. However this solvent would have no effect on permanent marker.
Acetone would dissolve the permanent marker and also it would mildly dissolve the original paint. I wouldn't mind having a thin layer of the original paint removed, but the problem was that dissolved marker would mix with dissolved paint and that would leave ugly stains in the upper layer of the paint.
Also I tried windscreen washer concentrate which is water plus isopropyl alcohol plus some unrelated chemistry. It would dissolve the marker and have no effect on the paint but the dissolved marker would burr and immediately adhere to the surrounding paint so I had to wipe the burrs immediately and still would have minor low-contrast stains.
Then I figured out how to combine acetone and isopropil alcohol. The deal is that the marker trace is rather thick - the lower layer is in direct contact with the base and the upper layer is not in contact with the base, it is separated with the lower layer. The alcohol wouldn't remove the lower layer because of it good adhesion to the base, but it would dissolve the upper layer. The acetone would dissolve everything and mix it int ugly mess, most of which was the marker ink upper layer. So I used alcohol to remove the upper layer, and then acetone to remove the lower layer together with some original paint. That did some damage to the original paint, but it looks minor especially compared to the high-contrast graffiti drawing removed. In my opinion that was a win, but not an epic one.