Some preachers use some baseless stories in the mosques and elsewhere specially to motivate Muslims to perform the religious duties of Islam. People are usually easily motivated by such stories. There are unbelievable stories about Pirs (religious leaders) who are thought to have done miracles during their life time and also after their demise. What should a sincere Islamic preacher do in such a situation especially when, if we directly oppose those fairy tales like stories, then our integrity as sincere believers will be questioned. We can be regarded as persons creating dissension in the Ummah.
This is indeed very unfortunate that some people try to persuade others to do good acts and avoid evil through narrating such concocted stories. The listeners also do not bother to think that they are being fooled by accepting things which they wouldn't believe in if only they were to deliberate.
The best approach to preaching is found in the example of the Prophet (sws) who used to recite the Qur'an before his addressees. The best discourse and the ultimate source of guidance is the Qur'an. We should try to present its message in a clear and cogent manner. We should try to inculcate all the good qualities in people through teaching them the Qur'an.
We understand that a believer is always supposed to uphold the truth and should try to replace the wrong with the right. Of course he must do it with wisdom and should not create disorder. While we can have a dialogue with such people and convince them about their folly, we can adopt a positive approach in this regard. Since the Qur'an is the book which is revered by all Muslims, we should, instead of telling such people that they are wrong, explain to them that we should read out to people a translation of the Qur'anic text. In this manner, we can achieve our objective without being labeled as insincere.