Question: Is it disbelief to say, "One must not say 'Ameen' for an unacceptable du'a"?
ANSWER
First of all, let us discuss whether there is a du'a (prayer, supplication) that will not be answered. For example, saying, "O my Lord, appoint me as a prophet," is an unacceptable du'a. Supplicating that way is contrary to the command of Allah, and the word Ameen (it means "O my Lord, accept my du'a") must not be uttered for such a du'a.

Besides, the word Ameen must not be said following the du'as that are against Allahu ta'ala's divine habit. An example of it is supplicating by saying, "Do not make me die" or "Make me an angel" or "Make me a female." Moreover, it is a sin to pray [to Allahu ta'ala] to put you into Paradise without you having to perform acts of worship.

Supposing a person gives alms from property that is haram and the poor person given the alms says "May Allah be pleased with you [for this charity of yours]" or "May Allah accept it," even though he knows that the property has been earned by way of haram, and thereupon the almsgiver or a third person says "Ameen," in that case both or all three of them become disbelievers. (Birghiwi Sharhi)

It is an act of disbelief to perform an act of charity, e.g., to have a mosque built, out of certain wealth that is known to be haram and to expect thawab for that act. (Radd-ul-mukhtar)

As is seen, there are du'as that will not be accepted and after which the word Ameen should not be said. Therefore, saying "One should not say 'Ameen' for an unacceptable du'a," does not cause disbelief. Nevertheless, it is better not to say such things.