In order to combat graffiti, there are two main options for treatment, a barrier resistant to graffiti (i.e. paint etc will not adhere so readily), or a sacrificial barrier (removed, together with the graffiti). 

For architectural concrete, a ‘clear’ sacrificial barrier gives best results.  There are many different manufacturers, but one such product is Graffi Coat 1. This is a permeable water based coating, which is usually applied to the concrete using a low-pressure spray (i.e. pump-up garden spray).  It may also be brush applied.  It also helps to protect against dirt generally such as low-level landscape splash, particularly on areas that are regularly washed.  There is minimal colour change to the concrete when applied, although a slight sheen may occur.  It is claimed to offer 5 to 10 years protection.  The suppliers are Tensid UK plc, Scandec House, Pyrcroft Road, Chertsey,  KT16 9HP

Other materials are available including silane based products.

This concrete was treated with Tensid Graffi Coat 1
and then deliberately subjected to:

Car spray paint
Grey primer (brush applied)
Permanent marker pens
Spray adhesive
Coffee/blackcurrant mixture
Car touch-up paint (brush applied)
‘Shoe’ rubber

This is the above panel after removal of the Graffiti Coat barrier some 3 months later.  (Although not apparent from the photos, the concrete colour remained consistent).

After removal, re-application of the barrier is usually advisable.