This is one of the facts about hackers that is
not very comforting.
Even after you take precautions to prevent further attacks, unless you thoroughly clean off the hacker's little access files - he can still easily access your system.
Another not so nice fact is that, unless you really learn (or already know) about computer systems, you probably will not either be able to detect, or stop the hacker. You can, however, get help. This is when you need to say,