Is it permissible to supplicate for the suitor to be turned away and to reject marrying him without being frank with him about that?

It is permissible to supplicate for the removal of an unwanted suitor, and it is more proper to ask Allah for what is good. One should weigh his good and bad qualities and perform Istikhara (prayer for guidance) to Allah. If his religiosity and character are satisfactory, then praise be to Allah. Otherwise, the correct action is to annul the engagement, because marriage is a contract whose harm endures, so a woman should take precautions for herself.