I set up a zero tolerance graffiti policy , back in the day , which basically allowed a tagged train to complete its journey (rather than cancell say a peak trip - better a train than no train) - but for the set to be taken out and cleaned asap - even if it meant much swopping around of sets. Graf vandals thrive on seeing their creations out on the road , so spoiling their cred in this way was an attack on them and their egos. Another thing to do was to spoil their "art" by putting a streak of white paint across it (until it could be cleaned off)
Policy worked well - with only a few major hits - at Xmas and New Year we put on a load of extra security and did not outstable at vulnerable , quieter locations - thus we didnt get a single attack - unlike some other TOC's , who got a lot of trains painted.
In terms of internal tagging - we used to clean if off asap - special measures being taken to do so as part of normal terminal cleaning.
These were tips I picked up from the New York subway - which in the 1980's cracked the problem - I spent an afternoon in Coney Island yard (on a personal holiday I hasten to add) , seeing how they managed the "clean car" policy.
BTW - many of the vandals were quite well off , middle class types.
The damage to the company image must be prett substantial I would imagine, although for Northern perhaps that isn't really an issue?! Somehow I doubt you'd see an SWT unit pulling into Waterloo looking like that! 
, EMT had to operate their services in various liveries (CT/TPE/ATN) as there was no other 158s for their Liverpool - Norwich service!
