An isolated body that doesn't exchange any angular momentum with the outside universe will never stop spinning (by conservation of angular momentum).  There is no way to absorb angular momentum within the body in internal degrees of freedom; the angular momentum must be transported away if you want to stop.

For example: if something is not rigid, you could have what's called "differential rotation", where maybe the crust of the planet (say) is spinning one way, but the core is spinning the opposite way.  Ignore the problem of how to make this happen, just imagine it does.  There's still some total angular momentum.  And if the initial angular momentum was not zero, the total angular momentum must still be nonzero.  You cannot dissipate angular momentum within an isolated object.

Tidal locking happens because the angular momentum of the earth's *spin* (in this example) is transported to the angular momentum of the moon's *orbit*.  To emphasize, it gets transferred from a spin to an orbit.