It sounds like slugs or snails -- and one of the best treatments for these critters is to use a slug/snail bait. Look for baits at your local garden center or favorite online retailer; there are traditional and organic types available. 

 

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I've also heard that mulching the soil around your plants with coffee grounds can help deter these critters, but I've not tried it myself. 

There's an old-time remedy that you can dig a shallow pan of beer in your garden so it's at soil level. A number of slugs/snails will crawl in and drown. This can provide some control, but it's not usually effective in large gardens or if you have a large population of slugs/snails.