Threatning to blacklist my under age son(16 yrs)

in sept 2021 they made my 16 yr old Son to sign a 12 months contract without parent concern. when i took it up to them 2 months later , they would not even produce the contract , they kept telling me to leave my email adress, but i would receive nothing on my email. I also attempted several times to contact their head office online from their website and also via emails , which will then direct me to the branch manager. They kept promising to resolve the issue but nothing happened. I told them that the so called contract was not valid since parents did not give concern. they did not respond to my emails any more and they have now handed him over to dept collectors - they keep sending sms and are now threatning to blacklist his name.