How to stop next doors dog digging under our fence?

[Mytwistedimagination]

First off, I don't dislike dogs :).

But I do dislike my ndn's dogs! They have three or four (not sure if one visits) and one in particular is a little bugger. Sustained barking at any little noise from my side of the fence, even the sound of watering the plants. It has dug through and got into my garden twice, and can't be caught without the owner chasing it round. I've got some plastic anti dig board at the small gap along the bottom of the fence, but it just digs under it. The two original escape holes have been blocked off by the NDP, but there's another two holes being dug which I've mentioned, and despite saying they'll block them off, they've done nothing. At the moment I've got large plastic toy boxes covering what I can on my side, but the damn dog just digs further along, and has already dug away a huge part of my flower bed.
Is there anything I can put down which will deter the dog from digging? I tried pepper but it had no effect :). Any barrier gets dug around or under. Ndn isn't interested.
Any good ideas?

I would contact the council/dog warden equivalent in your country.
In the UK, I would definitely contact dog warden - they may contact the owner, and if it gets in your garden they could catch the dog and then charge the owner the release fee which will hopefully teach them!

If its a rental property then I would speak to landlord.

Technically in the UK you could make a civil claim for destruction of property..

In addition in most countries dogs are property, so the minute you pick up the dog and take it indoors, let alone to the pound you have stolen it and if you drop it off somewhere you have 'dumped' stolen goods. - Except the dog would have been 'straying' and therefore it would not be illegal to take the dog in and report it. That's like saying if you pick up a wallet you find in the street and take it to the police station you are stealing it? As long as you report it to the proper authorities (the council in the UK) then that's fine.