1. Limit the number connections to SQL Server database.
2. Restrict connections to SQL server from tools like SSMS (or) You dont want non-sysadmins to connect with SQL Server except from application.

We can acheive all this by creating logon triggers.

Sample 1
=======
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;

Sample 2
=======
This trigger filters restricts other applications to access the database other than the  APPname (replace your application name with APPname)
Declare @role numeric(1)
select @role= IS_SRVROLEMEMBER (‘sysadmin’);
if @role1
begin
if (SELECT COUNT(*) FROM sys.dm_exec_sessions
            WHERE is_user_process = 1 AND
              program_name’APPName’ and
                original_login_name in (select suser_sname()))>0
Rollback;
end;

Note: we can use sp_configure ‘user connections’  to limit the number of connections to SQL Server.