How can I fix this problem?

I dated my coworker 3 years. We said we loved each other but overall were more casual but he called me his girlfriend. He left our job for a new job about 2 years ago. We broke up 6 months ago. I've moved on with a new boyfriend. But I really wish he never came back here. he's working here a few times per week. Its a good government job, so i would rather not quit. I've asked him to stop working here because emotionally its too hard to see him. Why can't he just work overtime at his current full time job?
How can I fix this problem?
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