You can get a systemic insecticide such as Malathion or Lathion. It enters the plants sap through the roots and poisons any bug that tries to feed off of your plants. I'm pretty sure this is what they use to protect tomatoes and other vegetables from being eaten away.
You have to be careful when using these products though as they are poisonous to more than just animals and are not the most environmentally friendly products if used incorrectly.
I'm pretty sure I've also seen products in garden centers that focus on snails too but I've never had the problem myself so I never looked closely. The best bet is to go to your local (BIG) garden center and ask them because they'll know.
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