You can remove a sub-account from your Master account by removing it from your dashboard. To do this, go to the dashboard from [Website Management > Billing & Support > Account Center], click on the account you would like to remove, and a management tray will slide out. From there, click on remove account.
By removing the account here, the sub-account will be set to expire at the end of its billing period. You will then need to login to the removed sub-account directly and update the billing information within [Website Management > Billing & Support > Account Center] to continue this website past the next billing period.
Note: If you remove, expire or unlink all sub/managed accounts from your Master Squarespace account, you will no longer qualify for discounted billing. The discount is provided to Master accounts that are managing at least one paid sub account.