Posted: 06/10/2013 at 11:46
It depends on the weedkiller. If it's systemic you need to wash it off or prune it off immediately after spraying to prevent it getting to the roots and killing the whole plant.
Either way, I would take photos of all the dead or damaged plants and then prune out the dead wood and maybe erect a fleece or net wind break around the ceanothus to protect it from biting winter winds. Give it some bonemeal lightly forked around its base to help encourage roots over the winter.
Get in touch with your local council's community welfare officer about the problem with your neighbours to see if they can mediate or issue warnings and, maybe the police too. Wilfully damaging other people's property is a criminal offence.