How do you deal with that?
Try a contract. Contracts are part of adult life. Offers of employment, mortgages, rental agreements, car leases ... all are contracts. With that understanding, you can craft something mutually acceptable with your teen.
1.) Identify your issue: Spending money, use of car, social schedule, homework.
2.) Each party lists their responsibilities: "I will..." "I can provide..." It is best, where possible, to phrase these in positive terms.
3.) Brainstorm consequences of non-compliance. Again, this is not "treating him/her as a child." Most contracts have a penalty clause; many domestic contracts also include a dispute resolution section.
4.) Print two copies: One for the teen to keep, and one to be posted someplace central.
Tips for parents: Though you'll have an agenda for this document, you can't write it single-handed. Like any contract, it will be negotiated, Don't be too generous; if you make a concession, make sure your teen understands that you have done so. Don't make two in a row if your child hasn't made one yet! Concessions are traded -- no one gets everything they want, but, if everyone cooperates with the process, everyone gets most of it.
Welcome to the Adult World.