General remark: the tricky part of logins used on multiple sites is that they mostly all have their own ToS/PP, so deleting an open account may not mean deleting everything submitted to the sites you used that account for. In those cases it may be really worthwhile to actually read the ToS/PP, for a change.
From their site: Contact support if you need to () permanently delete an identity. That is: [email protected]. Also from their Tos: a terminated account may continue to exist for up to two business weeks before such cancellation takes effect.