I need this info as well as some little ..... used a spray paint to leave his name (how smart that was! ) on a similar type (plastered) wall beside me.
You could try a pressure washer I used my own to take weathersheild paint off of my garden wall previous owner decided to paint the tops of the wall white so I tried the pressure washer an lo and behold it did the trick fantastic job.
If that does not suit you then try HomeBase they have a range of paint strippers for removing this kind of thing.
Spray on graffiti remover,( its celulose thinners) from woodies or a such, spray on in small areas, the wire brush it.( bad for skin and hightly inflamable)
Option two
if the graffiti is oil/spray based, use panit stripper, wire brush then powerwash the whole lot.
If emulsion or water based, hot powerwash with wire brush.
I have the same problem, can anyone who tries the suggestions above, with the exception of the whitewashing, and successfully removes the paint post back and let us know how?
Thanks from me too. Will report back on how I get on too. Now if only the rain (in Dublin) would stop for the weekend and I might get to try out the remedies.