I think Kind is dead on. That is how you let her know you know about the affair. (Remember what R2C said, do not tell her what you know, or how you know, only that you know!)
Next time she tries to initiate sex you stop her and say what Kind said above. Now she knows you know. When she presses you, you stop the conversation with a simple "I know what is going on." And then walkaway, leave, go to sleep, say you are busy and cannot continue the discussion.
I am not sure if you have been told this information, but there are practical reasons you do not tell her what or how you know. WASs in affairs aren't prone to end the affair just because you know about it. But once they know HOW you know, they will go further into secrecy to keep you from knowing more. For instance, a lot of LBSs find out about their WAS's by snooping on the WAS's phone. So when a LBS gives in and tell the WAS "I saw the messages on your phone." Guess what happens? The passcode gets changed, and/or the cheaters switch how they communicate (they stop texting and use Snapchat, etc). There is nothing to be gained by telling the WAS how and what you know. So the best approach is just to say "I know". And leave it there.
My wife took this to the next level. She would install a messaging app when it was safe, message with the OM, and then not only delete the messages, but then uninstalled the messaging app that was used. Do not underestimate the lengths a cheating spouse will go to in order to hide their activities.
