If you've registered for a course and then decided you no longer wish to attend, you must contact your university to withdraw.
When have I officially started a course?
You've started a course when you've registered for it.
How do I unregister for the course?
You must contact the university to unregister.
Can't I just decline the course here on Universityadmissions.se?
The declining function is only for removing a course that you're not planning on attending. Declining a course makes it available for other applicants on the waiting list.
If you decline on your account, this information may not reach the university. You must contact them directly to let them know you wish to withdraw.