I don’t think there is another way, apart from starting over.
So, to recapitulate, you have:
CTRL+Z (in practice, you will not soon reach the Limit of 256 and who wants to press CTRL+Z so many times?).
Painting over the Brush stroke.
Starting over (highly recommended in many situations, where a few Undo’s won’t work).
(It is of course possible that there is another method, that I am not aware of.)
