There isn't much you can do to force a change in their relationship. You can establish curfews, household rules, and so on, but unless you plan to watch her every minute of every day, she'll find a way to break a rule that you put in place. While she is certainly not an adult yet, your daughter's old enough to make some decisions on her own.
The best way to end this relationship is to get her to end it, and have her look at it in a realistic, mature way. Emphasize the reasons that you are worried, and get her to really think about these issues.
It may be necessary to get a different adult involved in the conversation — a godparent, a favorite aunt, a school counselor somebody. The knee-jerk reaction of teens is "you don't understand me" and she may be less dismissive if these hard, important questions are brought up by somebody else.
Safe Sex. Is he wearing a condom every single time, regardless of whether she's taking an oral birth control? Sexually transmitted disease is a big risk in addition to pregnancy. And pregnancy can happen even when multiple forms of contraception are in place. Is she ready to raise a baby? Is she ready to have to face the choice of abortion or teen motherhood? Does she have money saved up in preparation for either option? Is her boyfriend going to be a good parent to their child financially and emotionally for the next eighteen years? Where are they going to live? What will her educational and career path be like when the complication of parenthood is introduced?
Legal consequences for him. Depending on the age of consent where you live, their relationship may qualify as statutory rape. Is he so committed to their relationship that he wants to run that risk? Is he interested in hanging out with her in non-sexual contexts until she's older? (While these are sort of questions "for him," ask her to think about them, think about what she'd want his responses to be, think about what his actual responses would be...)
Legal consequences for her. Dating a drug dealer isn't just a matter of having a rich, bad-boy boyfriend. Has he ever asked her to carry anything related to his business (drugs, customer money, paraphernalia like rolling paper or empty baggies)? Does he have any of those items in his car when she's riding in it? Even if it's just as innocent as "being in the car" there's a chance she'll be caught up in any subsequent criminal charges (and if she's got anything on her person, it's a much bigger deal). Legal trouble — fines, community service, jail time, a criminal record that could potentially follow her throughout college and job applications — creates many short- and long-term repercussions.
Will he nobly take all the blame for drugs and any associated paraphernalia? (The criminal justice system may not care.) Will he put all the blame on her and say he has no idea where the stuff came from? Will he just not bother saying anything? Will he pay for her legal representation as well as his?
General safety and ethics. In the course of his "job" he likely deals with a fair amount of dangerous people. If he's in a dispute with somebody, does she want to be around if things get violent? Even if she isn't around for those (hopefully rare) events, does she understand he's profiting from other people's addiction and sickness?