I'm not sure whether you are saying that it has been over a year since you split and your ex recently applied and was deemed the most suitable candidate for a position with your company. Or whether you split Mar 25 and he then applied and was the chosen candidate meaning you've been working at the same place over a year.
Either way, there really isn't anything you can do about it despite some of the advice you've received. If there are no documented incidents of abuse, and you have indicated that there isn't, you can't say to your employer "I don't like x and it's affecting my mental health" That is not what the duty of care is about 🙄
I doubt anyone would want to work with an ex they did not get along and that's completely understandable. You cannot expect an employer to NOT hire someone because you don't like them. If anything, you are going to be the one that ends up worse off. It is not professional to try and interfere in the hiring process and those saying that if you cannot tolerate the situation, you are the one to leave, are right. It sucks but only you know what you can tolerate and what you cannot. The company is not going to risk an unfair dismissal application on your say so.
If there are any incidents that have occurred since he began working at the same place, they need to be reported and documented. If there have not been, and you simply don't want an ex partner working at the same place as you, it is not going to be deemed as 'constructive dismissal' should you choose to leave. Can't believe anyone is giving you the idea it could be.
It seems a stretch, depending on when he actually got the job, to assume he's applied for, and interviewed for a job simply to annoy you. It's not a great situation for you, definitely get that, but probably not about you at all 🤷♀️