Has any planet ever been observed stopping spinning?
Rotation or spinning along a planet's axis is something that comes with the formation of the star system, where planet's orbit. Generally stellar systems and planetary system are fromed due to the collapsing of a dense interstellar clouds, which gives an sudden aid to the planetary system to get an awkward spin.There is no force that causes the planets to rotate. Since most of the planets are spherical in shape most of the rotation comes about from the conservation of angular momentum. Angular momentum is given by L=m*w*r^2 where m is the mass, w is the angular velocity in radians per second, and r is the radius of the circular motion. Due to conservation of angular momentum, if the radius of the orbit decreases, then its angular velocity must increase (as the mass is constant). As a result every planet has a spin.
No, none of the planets known as per discovery has ever stopped spinning or tried to stop its spin.