Garder's World, a programme that I very rarely watch, did a test on various anti-slug devices a few weeks ago, which included a circular collar of copper, maybe 3 inches wide and a foot in diameter, that you put round your plant, pushing it a couple of inches into the ground.
Obviously this is only useful for individual plants, and I've no idea if it worked, because I didn't see the followup. I can't even find any mention of it on the BBC gardening website.
I gather that Bill Oddie, during his Springwatch programmes, declared that black slugs don't harm our plants, it's all the brown and grey ones that do the damage. Never heard that before, but come to think of it, I did remove several black slugs from my radishes earlier this year, but there was never any trace of nibbling.
Alternatively, has anybody ever considered re-homing a hedgehog -
http://www.cwgsy.net/private/hedgehogrescue/Uists.html