Sure! And, I think the best rules-based answer you'll find is, "It depends."
In some cases, it's not unreasonable to think that a local god who fits the appropriate portfolio (and, generally, domains) might continue granting that power. Thus, while Sylvanus and Obad-Hai are not complete analogs of each other, they'll probably grant spells to each other's clerics.
Ravenloft is kind of an odd case, one which I don't know all that much about.
That being said, a cleric who grossly violates his code of conduct loses access to all class abilities other than the basics (BAB, Saves, Hit Points, Base Weapon Profs.).
SRD said:
Ex-Clerics
A cleric who grossly violates the code of conduct required by his god loses all spells and class features, except for armor and shield proficiencies and proficiency with simple weapons. He cannot thereafter gain levels as a cleric of that god until he atones (see the atonement spell description
Specifically, the cleric loses his domain powers.
This says to me that domain powers are a gift of the deity in question, and therefore are not "part of the cleric" as you say. If it were possible to completely cut the cleric off from his or her deity, then, yes, I'd say that they lose their domain abilities as well.
Keep in mind that this is, in many ways, like destroying a wizard's spellbook. It might be a legal tactic, but it's probably not going to win you any friends.
