If you want to close down your Project, you can either archive or delete it. Either way, you first need to zero the balance.
Go to your Project's page, click on the gear icon next to your logo and head to the Advanced page to access those options.
If your Project does not have any transactions or financial activity, you can delete it. Deleting a Project will remove its data, including memberships, payment methods, etc.
If your Project has transactions, you can archive it instead of deleting. This will mark the Project as inactive and prevent any new donations.