Emotions are a tough animal to tame. After all, the heart wants what it wants. A girl may not want to, but she may fall for a guy who already has a girlfriend. However, she should not scheme or plot to break up this relationship. Instead, she should find ways to get over the guy who has a girlfriend.
Understand that there are many other guys out there whom you can date. You should not look upon this guy like he is your only romantic option.
Talk to people who know this person. You may find out that he is not as nice as you think he is after asking others about him. Be aware that this can backfire, because he also may be a great person, and this can cement your affections. If this happens, refer to step 1.
Find and talk to some of his ex-girlfriends. These past relationships may not have ended well, and it can help you see another side of this guy that your affection may have blinded you to. Ask them why they broke up.
Get a hobby or take a class that takes your mind of this guy. Not only will this distract your attention, you could possibly meet a new guy to date while conducting these activities.
Analyze exactly why you are attracted to this person. It may be that you are not that attracted to him. It could be something happening in your life that caused this attraction. Also, examine your relationship, if any, with this guy's girlfriend. It could be that you are angry with this girl and your attraction to her boyfriend is a manifestation of that anger.
Avoid this guy at all costs. Then maybe over time your attraction will wane and eventually disappear.