Just an idea, but maybe good old fashioned elbow grease such as scraping might work. Also depends on the type of paint they used before on the walls. If it was an oil based paint, then some kind of chemical stripper might be an option. Sandblasting sounds a little dramatic, but if the paint is that heavy that might be your answer. Best of luck to you middie, plumber Tom

Sandblasting is the only way to fly.

You need to hire a professional to do the job.

No amount of elbow grease or chemicals will remove paint once imbedded in a masonry product.