I'm also interested in the broader ethical question - this vandalism isn't the nastiness I've seen, but were it much worse, would it be a terrible thing for me to log off and fix the problem? Can banned users have a recourse to IAR if they're accused of breaching their ASBOs? ( http://en.wikipedia.org/wiki/ASBO)
As long as it's completely obvious vandalism without the slightest possibility of anyone (including the vandal) thinking otherwise, then I would be happy to turn a blind eye to banned users reverting it. Reporting it to someone else via email, IRC, whatever, might be better, though.