Evidently the storage area that you have for your canna rhizomes is too warm. The ideal storage temperature is about 40 degrees F, which will prevent them from sprouting. Placing them in an old refrigerator is one option. I keep my cannas in an insulated, unheated garage until temperatures there dip to freezing. Then I move them to a cool corner of the basement. Unless the plants also receive ample sunlight, the developing shoots will stretch and be weak. If the rhizomes have long shoots at planting time next spring, use caution in planting them to avoid damaging the shoots. You may need to pot them up early indoors and grow in a protected location until they can be planted safely in the garden.