To start, prepare mentally. Since you had contact with a married man, so he was dear to you. As they say: "tear off the bandage dried up blood, but quickly." You need to try to convince myself that it is a sin, but if you are a believer, then you can just find the priorities in your life without married men. After all, nobody likes to be second, especially in "matters of the heart".
Further action is preferable to the conversation "eye to eye", so to speak, to explain everything. Don't need to tell him about what you pretend to be his wife, because he may think that you have to it was only materialistic goals. Just stands to prove to the man that you have completely different paths in life that you just are worthy of happiness, but, alas, not with him. In this conversation it is important to show your dominance and try not to deviate from the topic. This will help to prove that you are determined.
If after this conversation will begin regular calls, texts, emails with declarations of love, don't ignore them and just keep your distance in communication. It happens that the woman immediately chops off all means of communication: blocking a cell phone number, deletes e-mail, change all the contacts known to him. This is not desirable. Just such a situation a man may perceive it as a game and, of course, want it to be a winner.
Of course, there are cases where this secret relationship develops into marriage, but such examples are rare. To stop relationship possible, but only if very strongly to want it, and understand that someone who cheated once can do it again.