She can continue to do what she wants-- remember, you can't control her actions. But you've stated a boundary: artifacts of an extra marital relationship have no place in the home you currently still share together. And if you see them, you will remove them.
And who cares that she was "unhappy" to remove the cards. What did you expect her to say? "Oh, you're so right, OD, how awful I've been to you"?? Sorry, you are not going to get that!
Part of boundary setting is also limiting the emotional grip our WAS have on us. Don't let her reaction throw you. Hang in there and yes, she is right that you need to focus on your son. (Someone here did warn you about timing, no?)
