Idea: Have specific areas within an urban environment to permit graffiti as an art form and to keep other public buildings clean.
Enforcement: No idea how. The potential LE officer in me is cringing at this idealistic notion.
Banksys 23-foot high Stained Window piece (first image in this post) is a collaboration with the City of Angels public school in Los Angeles. Students were encouraged to write tags on panels erected in their schoolyard before Banksy adapted them.