media markt- Refund

Unfortunately, consumer rights are not well publicised in Belgium, so "general" advice is going to be difficult.
I don't even know if you had the right to cancel in he first place.
Do you know anyone who's a member of Test Achats? If you do, get them to ask.
You could also ask the credit card company, but I'd expect nothing more than a "go screw yourself" from the call-centre. You never know though...