This is not a problem of Matlab, but a feature of the operating system. So this question is actually off-topic.
Open the control panels -> "User and Groups". Choose the correct user on the left side. Select "Login objects" (or similar name - "Anmeldeobjekte" in German). Using the + and - signs you can enable/disable auto starts.
You can include a program to the auto-start directly from the dock also.
Or is Matlab started, because it is open when you shut down the computer and you have selected "Open windows at next login in" in the shut down dialog?