Pimsleur STR*Learn Spanish Received five extra sets which I did not want. $344. No call back when I complained to answering machine. Internet

In November I sent for a set of Spanish Language CD's by Pimsleur as a gift for my son who was having trouble with Spanish in college.  I paid $68.80.  Since then I have been sent five more sets at $68.80 each (a total of $344.00 more).  I thought it was my fault because I didn't read the fine print.  I tried to call them at the number on my billing to tell them to cancel whatever I unknowingly did.   It was an answering machine with a man's voice.   I left my reason for the call and left a return number.  I did not get a call back.  I thought Pimsleur was a name to be trusted.  I feel extremely foolish now for being a victim of this.  I will not pay for another set.  However, if I refuse to pay, I worry about my credit rating.  What can be done?