How can I remove graffiti from painted walls?
I think your best bet is to just paint over it/re-paint the wall.  On exterior concrete/brick walls it is easy to power wash or sandblast it - the goal is to remove everything. However neither of these will work inside - aside from the mess they'd create, they'd probably take more then just the paint off your walls!

You might find some chemicals to help remove it, but because spray paint is paint afterall, you will undoubtedly damage the paint underneeth it.  Permanent Markers can often be removed with Isopropyl alcohol if the markings are relatively new - you might have some luck with this on the spray paint too. However, with markers, generally speaking, the longer you leave it, the more difficult it is to remove.

By the time you get this stuff off, fix the wall, re-paint the parts that need it, you'd probably would have spent more time then just repainting.