sp_denylogin (Transact-SQL)
Applies to: 
SQL Server
Prevents a Windows user or Windows group from connecting to an instance of SQL Server.
Important
This feature will be removed in a future version of SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature. Use ALTER LOGIN instead.
Transact-SQL syntax conventions
Syntax
sp_denylogin [ @loginame = ] N'loginame'
[ ; ]
Arguments
[ @loginame = ] N'loginame'
The name of a Windows user or group. @loginame is sysname, with no default.
Return code values
0 (success) or 1 (failure).
Remarks
sp_denylogin denies CONNECT SQL permission to the server-level principal mapped to the specified Windows user or Windows group. If the server principal doesn't exist, it's created. The new principal is visible in the sys.server_principals catalog view.
sp_denylogin can't be executed within a user-defined transaction.
Permissions
Requires membership in the sysadmin fixed server role, or execute permission directly on this stored procedure.
Examples
The following example shows how to use sp_denylogin to prevent Windows user CORPORATE\GeorgeV from connecting to the server.
EXEC sp_denylogin 'CORPORATE\GeorgeV';