Você pode tentar os acionadores de login: link
O TechNet sugere que:
You can use logon triggers to audit and control server sessions, such as by tracking login activity, restricting logins to SQL Server, or limiting the number of sessions for a specific login.
O primeiro exemplo de código limita um usuário a 3 conexões (citando o TechNet):
USE master;
GO
CREATE LOGIN login_test WITH PASSWORD = '3KHJ6dhx(0xVYsdf' MUST_CHANGE,
CHECK_EXPIRATION = ON;
GO
GRANT VIEW SERVER STATE TO login_test;
GO
CREATE TRIGGER connection_limit_trigger
ON ALL SERVER WITH EXECUTE AS 'login_test'
FOR LOGON
AS
BEGIN
IF ORIGINAL_LOGIN()= 'login_test' AND
(SELECT COUNT(*) FROM sys.dm_exec_sessions
WHERE is_user_process = 1 AND
original_login_name = 'login_test') > 3
ROLLBACK;
END;