How can I stop dreaming about my ex?

I dream about my ex-boyfriend nearly every night. I was in love with him but he broke up with me almost two years ago. I was having occasional dreams about him but he recently started working at the same place as I do, his cubicle is in plain sight and I cannot avoid thinking about him. I want to because the dreams are painful- I still have feelings for him but he treated me badly, I don't want to date him or anybody else. I focus really hard on work during the day, but these dreams at night are still happening, and I hate it! Help!