I wouldn't try to antagonise him by threatening to report him to the tax office as that could just lead to things escalating.
Why not just live with it for a few weeks then next time you see him / her outside go and make conversation about something else. If you strike up a rapport with people first it's amazing how friendly they can be.
If I was your neighbour and knocked on your door saying "I don't like where you've parked your van, and I'm reporting you to the tax office" I'm pretty sure you'd be hacked off and dig your heels in, but if I came round, introduced myself and offered to go out for a drink sometime then mentioned in conversation about the van you'd be more inclined to move it.
Having just read that it sounds very patronising but I hope you get the jist - I'm trying to help even if it seems a bit long winded!!
Good luck in getting them to move it,
Dan