Is it permissible to ostracize a person for committing a major sin (such as sodomy and sexual harassment) that occurred a few years ago?

Accusations of immorality are not permissible except with clear evidence. If a person repents, it is not permissible to shun them, because repentance erases what came before it. However, if they persist in disobedience, then shunning is permissible, though it is preferable to consider what serves the best interest. If shunning is the more beneficial course, then it is preferable. But if maintaining a connection with admonishment is more beneficial, then let that be the approach. If what is meant by "disputing" is filing a lawsuit, then there is no objection to that, because rights are not forfeited by the passage of time.