Fill a Jerry Can 1/2 full of diesel, mix it with a couple gallons of used motor oil, pour it all over the slugs and anything they might want to eat and set the whole mess off with a blowtorch.

Then go down to the store and buy a sack full of veggies.

Seriously though, there's that grey goopy stuff that comes in an orange bottle. You can squirt a line around your garden and any slugs coming across will turn to goo.