The power delegated by Iluvatar

[Telchar]

Westerly Wizard said:

It seems to me that Saruman for certain loses more than just power in his incarnate form. After Grima kills him, he no longer has the power to retake a form, which he certainly would have had before he was sent as an istar.

Do we know this for a fact? Who sais that the spirit of Saruman wasnt able to take some kind of shape in some later age? We know for a fact that it took Sauron a long time to regain power and physical shape.

Cheers T