Sorry for what you are going through
You ask "how can i stop them talking when his job means he has to"
It's neither your problem or your responsibility to "stop them." It is your responsibility to keep your wedding promises, which you have done. It is your husband's responsibility to keep his, which he has spectacularly failed to do. It is he who has ruined this.
He has shat all over your marriage and deserves to lose everything. He needs to be asked to leave so that a) you can think about whether or not you want him back and b) so that he can think about what he has chucked away.
If your husband wants to save his marriage then it is his problem to stop himself talking to her. He'll probably have to get another job. "Impossible!" I hear him cry. . . well then you're set to lose your wife, your house, your kids and xx% of your income, mate. Should have thought of that before you dropped your trousers.
You need to start getting angry. If you don't wish to confront him just yet I would suggest seeing a solicitor in the meantime.
This woman from work, btw, are you confident she couldn't be the OW trying to trigger a confrontation?