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.
Be logged in, use the deletion link. As you will see, there is a choice (checkbox) between deactivation and deletion. Pretty neat.