BLAST! Fitness Isn't illegal to harass a minor over money? Burbank Illinois

My mother, sister and I were expecting it to be a normal day. We were getting memberships from a gym named BLAST! located in Burbank, Illinois and my father was supposed to pay for them. We got the “platinum” membership which meant we had more access to equipment, personal trainers, etcetera. My father paid the fees, we signed the necessary documents and were told we would only be charged $20 per membership. All three memberships was supposed to be $60, with tax about $64. My mother, sister and I had only been to this gym a few times when they tried to charge my father an additional $300! Their explanation was that we had to pay a fee for our memberships 6 months in advanced and an annual yearly fee. This what not explained to us prior to us agreeing to go to this gym. They promised that we only had to pay $20 a month per membership and that would be all. My father paid the gym a small percentage of what they were asking for hoping that it would appease them. For a while, it did. Until they demanded that we pay an additional $150 PER membership; that's $450 for something we were only supposed to pay $20/month for! Deciding that the gym wasn't being truthful, we stopped going and had our memberships cancelled after only being members for a month. A few weeks later, BLAST! started to call my phone every day. Being only seventeen, I thought that if they wanted to speak to someone about the finances, they would call my father. His number was on file and he was the one who signed-off and paid for the memberships. After a while, they stopped calling me. I thought it was over. Recently, I have been getting phones calls from a company that works with this gym. The company's name is Seas & Associates. I have spoke to one of their workers. She was rude and threatening. She made threats that if we didn't pay, my father's credit would be ruined. I told her that I was a minor and that she should talk to my father. My father explained to her that we haven't been to that gym in MONTHS and that she being very unprofessional by calling me. He guaranteed her that we had cancelled the memberships. The calls are still being made to me daily, Monday through Friday. They have sent my sister, who is only 16, letters saying that SHE owes them $150. They have my father's number on their file. Why are they calling and harassing teenagers? If it's that serious, why won't they take my father to court? Any reasonable/legit business would take legal actions. They know that we haven't done anything wrong. They are the ones who are harassing over paying for something we don't use and haven't used in months. This is absurd behavior on their part.