It starts spinning because of the impact with the ground.
In order to prevent this, You need to set the Rotation X, Y, Z to 0 in each loop.
B.T.W - There is a small bug that prevents the setting of Velocity when the object is not moving.
Until we resolve this issue, You can use the Force command with X,Y,Z=0 and power=1 as a workaround.
We have updated the example (clear your browser cache):
www.gamemaker3d.com/player?pid=0164923
Regards,
Cyberix3D team