I want to get my certificate

First year I didn’t have NSFAS but WSU managed to register me and now I owe them but my problem is that they are sending me emails and sms asking for their money and it’s hard for me to get a job because I can’t get my certification because of the amount I owe them. It’s been 3 years now I graduated. I got any sms that the money I owe them it’s increasing. My questions are why and how I’m gonna pay them if I can’t get my certificate to look for job? I tried to talk to them at least they should make a copy of my certificate so that I’ll be able to get a copy. Can you please help me.