Of course, you can easily close your joint account again (although this would make us a bit  sad).
To close the joint account, the balance has to be zero (do this e.g. by transferring the leftover amount to your private accounts). Also, there must be no open card bookings on the joint account. Once this is done, one of you can simply close the joint account via the app by going to the «Profile» tab, clicking on «Accounts», «Joint», and «Close joint account».