what van do you have?
one simple thing to do is buy some alarm stickers for a few quid. better to fit a decent alarm of course but better than nothing. then you can get deadlocks and others like the ones posted. You will not stop them especially at night parked up but you can make your van harder to get into than the next one down the road and hopefully they will do that one over instead.
my old company had deadlocks ect... on its vans and on one job there was a large number of vans done over the same day. they missed out all the vans with extra security and went to the vans without.
they normally try and park up alongside you van so they can open there side door and attack yours without being noticed. so one tip is to keep your side door away from where they can park. I always park my van a few inches away from my fence so they cant get down.
if you have a mark one expert then the best solution is to weld the door shut 🙂
edit : forgot to ask if you reported it to the police??