Ace - I found myself in a very similar situation about 8 or 9 months into R with my wife. That is when I learned about the loose term "Mental NC", or "Mental No Contact".
We had mutually agreed to block the OM, along with his friends and family. At one point, I found out that my wife had unblocked him, and was looking at his pictures on facebook.
I was floored. When I initially confronted her, she was reluctantly remorseful, but then when I gradually became more and more upset, her defenses went up, and she basically had the attitude of "I'll do what I want." Her argument was that she wasn't contacting him directly in any way, so NC wasn't violated, in her opinion. Plus, she's an adult, and she can look at whatever she wants to on the computer.
Once again, I was floored, and at a loss for what to do next. The more I thought about it, the more I realized that this did break NC. By looking him up on the computer, she was essentially bringing pictures and life updates by him into our home, into her head, and into our relationship. It may as well have been like her holding on to a living and breathing scrapbook of OM, that was automatically updated a couple of times a week.
I addressed this with my therapist, and what she said was almost identical to what sisoon said here:
Then talk with your W and give her options - either meet your requirements or leave. You're not forcing er to do anything but make a choice - she's free to choose either option.
Additionally, my IC brought this up with my wife's IC (both of our IC's were colleagues, and we had signed off to allow them to exchange information). My wife's IC brought it up with my wife in her next therapy session as well.
So between me standing firmly on the ground with my requirements in place, and her IC being able to reason with her, we got past this huge stumbling block in R that came close to ending it all. To this day it is a trigger for my wife, where if something funky pops up on her computer, whether it is a spam advertisement, or a weird junk email, she shows me right away, just to ensure that I know she's not going out and looking at anything OM related, or that there is any form of contact, mental or otherwise.
Sorry for the length of the reply. I related so much to your story here, and I wanted to let you know that this is something that can be overcome by the advice that has been given here to you by the others as well.
Best of luck to you. Stand strong! We are here for you.