No, unless the lease specifically states that one party can unilaterally terminate it, a joint lease, like any multi-party contract (since that's what a lease is: a contract) requires the agreement of all parties to terminate. So generally, you and the car dealer/leasor would both have to agree to its termination. Of course, he can stop paying his share, and while you'd have the right to sue him for the money (for his share), you'd have to sue to get it. But in the meantime, if you kept up the payments, the lease would stay in effect. If you want the car and he does not, then best may be to see if you can resolve this amicably by entering into an agreement with this fellow in which you agree to make all the payments and to indemnify him for any costs in connection with the car, in exchange for you retaining sole use thereof.