You broke up with him but he won’t let you go?! How can he force you to have a relationship with you?? You need to set your boundaries clearly. In other words, interacting as workmates (yes, it will be uncomfortable) when you have to, but that's it.

IF he pushes it, then don't engage. A simple 'no' and withdraw. If he wants more, than that is *his* problem. YOU need to be very clear that 'no' means 'no'. Otherwise, he will keep trying to push it.