There are very different clinker. There are the hard burnt ones with a very smooth surface. There are porous or open-pored clinkers, also called Dutch clinker, and there is the so-called brick.
Basically, you can remove the graffiti on all types of clinker. With the very porous Dutch clinker, there is a risk that the paint will soak deep into the stone. Then the graffiti can no longer be removed without shadows. The most environmentally friendly way to remove graffiti is with the vacuum blasting system from systeco.