Dear [CORRESPONDING AUTHOR]:
Thank you for your further correspondence about this paper.
We have investigated the matter thoroughly. In our opinion there is clear evidence that [specify problem; e.g., a figure was manipulated inappropriately, the degree of overlap with another paper would constitute duplicate publication, etc.]. Therefore, ethical constraints and the policies of this journal require that we reject the paper.
I am sending this letter to all authors listed on the manuscript. [If the offense is severe, the next sentence may be added.] I will also be informing your institution [or: the institutions of all authors] of the outcome.