I think that Moose has a good point.
Women are absolute Mistresses of manipulation. I should know - I'm a woman and I only date women. ~laughs~
I have been in the position of having my girlfriend absolutely hate my secondary - so I can relate to your problem. It's incredibly tricky. She was the only other woman I had fallen in love with. I did not want to let her go. But, in the end, I had to choose between them.
You husband's ex certainly doesn't sound like someone I would want in my orbit. I'm taking your words on face value... I believe what you are saying, but know that there can be two sides to every story. However, I abhor cheating and whilst I understand *why* it happens, I also understand the colossal destruction it causes.
You have a number of options here. I'm assuming your husband already knows your feelings and that you've talked to him openly and honestly. If not, obviously, you'll want to be doing that
Option 1 - The Veto
To be honest, I don't think there's any such thing. Veto means "you cannot do this" - obviously, in poly, we cannot tell our partners what they can and cannot do. Essentially, what a veto really means is "I cannot be in a relationship with you, if you are in a relationship with her." Basically, you're giving him a choice. If you decide that this really is how you feel, I would put it across that way. This is what my girlfriend said to me and I made the decision to end it with my secondary. It took me a very, very long time to get over that and a long time to let go of the resentment - but, it was probably for the best and I do believe that now. Hopefully, if your husband decided to call it quits on the subject of her, he would also see that eventually too.
Option 2 - Change Your Approach
Sometimes, when there is no room to maneuver within a current relationship makeup, a change of perspective is needed. For example, in my poly relationship, it used to be important that everything fits and everyone gets along well enough. After a few situations, including a current situation of my GF dating someone I severely dislike, I realised that it doesn't really matter what I think. I could either leave, which I didn't want to do; or stay, but under a new frame of mind. I have adopted a more autonomous approach to poly and have distanced myself from my GF's relationships and from hearing about them. Basically... if she's going to say to me "if you want to be with me, you have to accept him", I have countered it with "if you want me to stay and accept him, you'll have to compromise and accept my dislike of him, accept that I do not want to hear about him and accept that I do not want to see him". This way, both people are compromising.
For me, this works best because I believe in promoting my partner's freedom and letting her make her own mistakes. If I 'vetoed', or gave an ultimatum, I'd either lose her, or end up in a relationship where she resents it for years. By letting go of the purse strings and basically telling her to go and do what she wants, I am able to let her play it out for herself. If and when it fades out, at least she'll have gotten it out of her system.... if and when it does, at least she will be feeling grateful for having had the chance to explore that. It could work this way for your husband. He's clinging onto what they did have, because he has unfinished business with her. I have a strange feeling that if he did start a relationship with her, it would not last forever....
Option 3 - Compromise and Compromise Again
You could go with the controlling approach. You could give the green light on their relationship, but under strict rules and guidelines. I do not think that this is productive. I think *some* general guidelines are good; but they have to be realistic. If there are more than about 8-10, I'd be worried. If they forbid emotions, or are stringent, I'd be worried. This approach is going to be the trickiest - it's going to take time and hard work. It's going to be incredibly strenuous for you. At least, that's the way I see it. I think it's possible that it could work - but I don't think it's the easiest path.
Incidentally, I DO think that this is one of those rare cases where it IS ok to say "sorry hon, I cannot manage this"... and let him make his decision on what he wants to do. This may just be my perception of your typing, but I am sensing a lot of blame towards her and a slightly softer approach towards your husband... because, to be honest, you love him, you've communicated with him, you've worked together. You haven't been able to do that with her.
My inclination, if I were in your shoes, would be to have a trial period of him dating her. Three months. You will not know how you feel until it happens. At the end of the three months, review it. He'll have spent some more time with her, which might be good for him. I still hang onto the ex I broke up with for my girlfriend. I still think about her. But... each time I see her for a catch-up, I drift further and further away from those 'in love' emotions .... because, out of the situation, I can see her flaws more clearly. It's possible that this could happen for your husband. One thing is for sure.... if he truly feels that he has to have her, that this is very important, he'll probably never get over her if you force his hand....
If it were me and he agreed to this three month trial, I would want a letter, or some form of communication from her. It's one thing for metamours not to get along - clashes happen. It's another thing to be expected to like the woman who caused you such pain. She basically has some grovelling to do and regardless of your behaviour, needs to show you an ounce of respect and compassion if there is any chance of you being able to cope with a relationship between them.