I think rejecting a part that is perceived as negative is the antithesis of processing, learning, resolution, and integration. I think the only healthy approach is to come to terms with a part, rather than reject it.
Before fully understanding such a part, one may reject it for poor reasons. This is a form of ignoring feedback from reality. After fully understanding a part, rejection won't happen, but change and integration might.
The only way I can see rejection being involved is if the part in question comes to a new understanding and 'rejects' some of its negative behaviours as no long necessary. But in this case it isn't the part or its past experiences that are being rejected. In fact, part of understanding why those behaviours are no longer necessary is appreciating why they came about in the first place, which seems to me to be the very opposite of rejection.