Put a rock next to the mushrooms at the side of your mushroom colony. Hopefully the mushrooms will move over themselves. Or the next time you see a mushroom dividing, quicken up the process by using a razor to make a clean cut along the spreading foot of the mushroom. Hopefully it would be easier to remove the 'daughter' mushroom then. Just try to gently ease the foot of the rock. You could try using a plastic card to do so.