SQL Agent Permissions - Job Level
First, make sure you have a user in msdb associated with their login.

USE msdb;
GO
CREATE USER username FROM LOGIN loginname;


Next, deny execute on sp_help_job and sp_start_job for that user.

USE msdb;
GO
DENY EXECUTE ON dbo.sp_help_job TO [username];
DENY EXECUTE ON dbo.sp_start_job TO [username];


Next, create two wrapper stored procedures that execute as owner and call the above stored procedures with that hard-coded job_id. Here is the start version, the help version will look amazingly similar.

USE msdb;
GO
CREATE PROCEDURE dbo.start_that_job
WITH EXECUTE AS OWNER
AS 
BEGIN
  SET NOCOUNT ON;

  EXEC dbo.sp_start_job @job_id = 'ACE1...';
END
GO
GRANT EXECUTE ON dbo.start_that_job TO [username];
GO


Now, they'll have to call that stored procedure directly, instead of using the UI.

Test it out:

EXECUTE AS USER = N'username';
GO
EXEC dbo.sp_start_job @job_id = 'ACE1...'; -- fails
GO
EXEC dbo.start_that_job; -- succeeds
GO
REVERT; -- this is important