There are some ways of getting rid of them but they are tough bastards. You can google: snail trap and see if you find anything you fancy or there is a product named Ferramol, and I bet there are more, that you can sprinkle around your plants so the snails eat that instead of your crop, after eating they get full, retreat and die within some time after. It is also said to be "rainproof" for up to six weeks and non toxic, repeat every two weeks. Reading, after my last visit, I gather you are guerrilla growing but to others with the same problem can coordinate this with your neighbours to spread the area of effect. Read up on where snails can be found/hibernate to get them at an early stage.
Fruit Hill Farm --- The mailorder company for the environment friendly household, farm and garden
You can also take a jam jar and dig it down in the ground leaving some of the top above dirt level and pour some beer, yes beer, in there because snails are said to like beer or the smell.. Just keep in mind that you should change it every two days to keep the smell "fresh". I have not tried this method out for myself.
Good luck with the bastards
Dutchy