Once you decide to close your mining operation down there are two options:
1. You can leave your account as it is without signing into it or mining with it and it will be automatically deleted after one year of your inactivity.
2. You can request account deletion manually in the Settings > Accounts menu and confirm it via the confirmation link sent to your e-mail..
In either case, please check that all rewards were already paid to your payout address before deleting or abandoning the account.