I also agree that the power to rescind ostracism should lie with the Ecclesia, though perhaps with a simple majority rather than the super-majority required to ostracize them in the first place.
I also think it might be best to adopt something like the classical system Myroria brought up, where rather than proposing an ostracism of a particular individual directly you must first propose a vote on whether to ostracize someone and if so who. Alternatively, the vote itself could be the ostracism vote, but everyone is a candidate by default along with the option not to ostracize anyone and only someone who gets a super-majority of votes will be ostracized, so that way abuse of the system can potentially backfire with yourself getting ostracized.
Also, there could be a universal cap rather than per-person cap on how often these votes could be held, that is, the Ecclesia could only vote to ostracize someone every month across the board for example.
I think that when an ostracism is proposed the term for it should also be part of the proposal, that is one could propose to ostracize for just a month rather than indefinitely for example.