I think using weedkiller would be well dodgy and best avoided. As a follow-on to plotterpen I'd say - whatever you do, take before and after photos. Then you have evidence of impassability before and improvement after cutting. Have you involved your local councillor? They might be helpful.
------------- Always edited for sloppy typing - when I spot it!
Sorry to disappoint you all but I'm definitely not being held hostage by a forum-using madman!!
The outcome is pretty lame, just keep on dragging the bin through the weeds as I didn't want to use chemicals to sort the problem and I knew that the neighbour would do nothng.
However, the neighbour did apologise recently and I am now 'permitted' to park in the street outside his house without being sworn at!