A band of copper will keep them off your plants, or away from whatever you want forever.
Most slugs snails cant cross as it if its more than about 1cm wide, as its makes like a battery when they try.
Instant Electrolysis.
As soon as they touch it they just curl up instantly, then the come out in a minute or so, and cruise the other way unharmed.
I knew a French lady that "farmed" them in the city years ago as a protein source and its how she kept them in.
Cheap, sustainable and provided you purge them on bran diet for a week before harvest they are quite ok with white wine and garlic sauce....
No worse than mussels or clams.
If you have raised beds just a band around the base is enough to keep them off, and copper lasts for years and years, so its a good investment.
Doesn't work 100% well in really hot dry weather(no snails then anyway) but in humidity its awesome.
Next time you see a construction site raid the bins for water pipe and hammer it flat, wrap it around the base or the beds, tables/bench legs, put a strip across the ground leading into the greenhouse or whatever.
Or buy it, but there is heaps of waste in construction and I'm too tight to buy stuff like that.
Cheap and 99% effective.
And no bad karma, which is a bonus! (provided you don't eat them that is)