Can planets stop spinning?

A planet cannot stop spinning due to the law of conservation of angular momentum; an object in motion can only cease spinning if a powerful enough torque is applied.

Tidal locking, in which a moon's rate of spin is lowered until only one side of the moon is visible from the parent planet, is the best example of how gravitational interaction with other bodies can change a planet's rate of spin by transferring angular momentum.

Our Moon used to be tidally locked, meaning that its rate of rotation is equal to its orbital period. The Moon used to orbit closer to Earth and spin faster, but the effect of Earth's gravity slowed the rate of spin by transferring the angular momentum to the Moon's orbit.

Planets can indeed stop rotating, but it would take a powerful outside force to do so, like a massive collision with another star or the pull of a much larger object.