MathOverflow is a means of communication between professional mathematicians. By standard professional ethics, these should be properly acknowledged in the same way that similar exchanges (informal discussions, email exchanges, circulated notes, published papers, as appropriate) are normally acknowledged, to the extent possible.
Note that we are currently working on means to ensure permanence of the MathOverflow database, which will facilitate proper and stable referencing of MathOverflow posts.
If you would like to ask how to properly react to a potential breach of professional ethics by a colleague, please ask on Academia.SEAcademia.SE since it's tangential here and you may get even better responses there.