How can I paint over paint stripper?

Some chav has graffitied my garage wall using a paint stripper or acid I think. I'm guessing, but it isn't spray paint. The white emulsion on the smooth rendering seems to have been eaten away.

I have re-emulsioned the wall several times and after a week or two the stripper/acid seems to eat through the new paint again. I need to neutralise the stripper/acid (and also the chav who did the damage.)

Any suggestions?