I don't have specific advise but, like LR, I was the one that broke trust - so I thought I would share how that looked from my side. (Reading GalaGirl's excellent posts makes me think that I actually got a number of things right - after my initial screw up - I won't rehash the whole thing - you can read about the details in my blog.)
His first response was to say "Never see/speak with him again." OK. While I didn't think that that was likely
to be the FINAL solution to the situation it was something that I could agree to "for now" or "for as long as necessary" or "forever if required." I suspected that he would change his mind but was willing to accept that the ban might
When Dude tried to contact me I would listen to the voicemail/read the email, tell MrS that Dude had called or emailed, and delete the message. After the 1st week MrS asked me to tell Dude the situation and tell him not to contact me again. I did (with an apology that I had put him in this situation).
MrS said that he didn't know if he could stay married to me, if he could live in our house, sleep in our bed. I told myself that if in 1 year he was still unhappy, still hurting, then I would let him go without a fight. No one should be chained to an untenable marriage. But for ONE FULL YEAR I would do everything in my power to regain what I had broken. (Meanwhile, in actual fact, 1 year later we were 6 mos into our cohabitating polyship and each of us happy and invested in "making it work"...who knew?
My challenge was how to support MrS through his period of darkness - him processing my betrayal of his trust AND mourning the lost of his best friend (Dude)- when I was the CAUSE of his pain. My approach was a mix of what I recalled from "The Ethical Slut" (I think - I can't find the section I am recalling) and some of the secular zen we both read. I just sat with him. In the same room...touching him if he could tolerate it. When he was in a place where we could talk my message was: "I know you are in pain. I know you feel I am the cause of it. I am sorry that I hurt you/that you are hurting. I can't undo what was done but I can sit here and share your pain, if you will let me."
10 weeks later Dude contacted MrS, MrS was in a place to listen, and the healing began....
So, how does that relate to your situation? I don't think that asking for "no contact" for now is unreasonable if that is what you need. 30 days (as suggested above), 40 days/nights (someone on this site's ban on contact after a breakup - sorry I am blanking on who), 10 weeks, 1 year, whatever...
I think it is not unreasonable to say "I don't know how I will feel later but RIGHT NOW I can't tolerate you interacting with this person...I will let you know if that changes, but I can't guarantee that it will. Give me 3 months and then ask me again, but I might say 'no'. Can you live with that?"
If someone is not willing to give you 3 months to figure out where your head is? Then they aren't making an effort in my book. At the end of the 3 months if you can't tolerate them seeing the other person, and they can't tolerate NOT seeing the other person - then you break up. No harm, no foul. You both tried.