Answer: I've had a similar problem with violets taking over my entire yard! Both calla's and violets grow from swollen roots that double as storage organs. Even small pieces can produce shoots and foliage, as you've discovered. Herbicides will kill back the tops of the plants but as long as there are carbohydrates stored in the roots, they will send up new shoots. Outside of patiently digging every last trace of roots from your garden, the only real way to control plants like these are to starve them out. If you can keep the foliage cut down you'll stop the process of photosynthesis (conversion of sunlight into energy). You may have to cut back the shoots to ground level every few days during the growing season before you gain the upper hand. But, if you keep the shoots cut down before they can produce leaves, the roots will eventually die out.
If the plants were just in one bed, I'd recommend excluding light by covering the bed with several thicknesses of newspaper and topping it with plastic to starve the roots out. But it sounds as though that's not an option in your situation.
Best of luck in eradicating the calla's gone wild!
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