2 suggestions, both organic..

Buy some copper tape - it's specifically made to wrap around pots to keep snails and slugs from climbing in. I've found it fairly effective. There is some sort of electical reaction between the copper the the slug's uhh - err- ooze.

If you want to make a moat, fill it with beer (Which is absolutely the best thing I can think of to do with beer... ) Snails love the ferment-y smell and climb right it - and drown. You have to keep it fresh - take out the deceased and put in fresh beer. It should be at least an inch deep dish with about 3/4" of beer. This also works well in the garden. Dig a small depression to match the size of the beer pan. Place the pan in the shallow and fill with beer as above. The steeper the sides on the pan the better.