This is a tough one...on the one hand, not finishing school is not a smart thing to do. But, you don't want to push her away either, since she can only go in other direction - to her boyfriend. If you get rid of the car and phone, she will rely on her boyfriend to help her. This is not what you want her to do.
I would suggest this -- she is an adult now, so she can make a choice. Her choice is...stay in school, and you will continue to pay her cell phone bill until she graduates. Perhaps, to ensure that she focuses on school as much as possible - you may want to lay off on holding her accountable for the car bills until she is done with her schooling, at which point she can pay you back in full or take on the remaining payments if there are any.
If she does NOT stay in school -- refuse to finance her cell phone bill and require that she get a job to cover her expenses. Maybe, again, hold off on her car debt so that she can feel some pressure of having to be financially independent without a college degree -- but with enough wiggle room so that she can freely explore the option of returning to school without feeling like she must work to support herself fully, you know? Perhaps give her 6 months or so of "wiggle room," and is she is still insistent on not staying in school, cut her off financially for good.
Unfortunately, at 18 years old, you cannot force her to do anything she does not want to do. The boyfirend living situation, aside from telling her clearly that youy don't endorse this, after that, the more you belabor the issue, the more she will be drawn to her boyfriend. Keep reminding her to use her birth control and allow her to explore openly with you, in discusion, potential consequences to living together, such as accumulation of unwanted debt, more intense relationship drama, pregnancy, even little issues that for an 18 yr old can be a big deal (always having to share a bathroom, living with a "less than clean" boyfriend, having no independence to go and come as you please, etc. Give her the freedom to make her own choices about the boyfriend living situation -- and choose to make no moves unless her boyfriend living situation ultimately leads to her quitting school or, if she stays, she is unfocused and her grades suffer. And, instead, focus on the more critical issue of doing whatever you can to keep her in school until she graduates, so that she can secure for herself a bright future. Good luck.