Adding, Removing, Re-Adding SQL accounts

Management Studio
[Server\Instance]>Security>Logins >> Add Login

SQL
SELECT SUSER_SID('<<username>>')

SELECT * FROM sys.server_principals SP where SID = <<user sid>>

Management Studio
[Server\Instance]>Databases>Object>Security>Users>User >> Delete User (Repeat as necessary)

[Server\Instance]>Security>Users>User >> Delete User

SQL
sp_droplogin <<username>>

SELECT SUSER_SID('<<username>>')

SELECT * FROM sys.server_principals SP where SID = <<user sid>>

Management Studio
[Server\Instance]>Security>Logins >> Add Login