I have been wanting to do the same as Rumen. I tried using the link you posted but It said that there was an error. I have been trying to cancel my contract or remove my credit card from the MAGIX support centre but I have no idea how to do so. Could I get some help please?
Did you try the 2nd link? Basically, read Purchasing details, they state that "send an email stating your customer/order number to: infoservice@magix.net". Please note that some cancellations don't happen immediately, depending on the payment option you chose.