Can planets stop spinning?
Planets can't stop spinning.
The law of conservation of angular momentum doesn't allow a planet to stop spinning. The only way spinning objects can stop spinning is if a sufficient torque is applied to stop it.
Planets rate of spin can change by gravitational interaction with other bodies which transfers angular momentum. The best example of this is tidal locking where a moon's rate of spin is reduced until only one side of the moon is visible from the parent planet.
Our Moon used to orbit closer to the Earth and it used to spin must faster. The effect of Earth's gravity slowed the rate of spin by transferring the angular momentum to the Moon's orbit. The Moon is now tidally locked and its rate of rotation is the same as its orbital period.