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