According to this website :
http://www.google.co.uk/search?hl=en&q=tagging graffitti&btn...
' A "tag" is the most basic writing of an artist's name in either spray paint or marker. A graffiti writer's tag is his or her personalized signature. "Tagging" is often the example given when opponents of graffiti refer to vandalism, as they use it to label all acts of graffiti writing (it is by far the most common form of graffiti).'

so I wonder if you can somehow produce or buy 'environmentally friendly' sticker tags which can easily be removed. It seems that one of the biggest complaints about graffiti is the difficulty of removing it....
Good luck...I saw some being cleaned off the Tate Modern (ArtGallery, London UK) last week-it had been put there by design though, and was being removed only because it was time to change the exhibition.

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Note added at 6 hrs (2008-09-18 18:48:48 GMT)
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Sorry, the web address above will only get you tp a Google page-this is the actual site:www.bozzle.com/id_graff.html