Care Credit/GEMB ransom/blackmail I must pay to get my credit score back El Paso, Texas

I opened a Care Credit account to pay for veteranarian charges.  I was paying as agreed for some time.  The first time I saw one of their $39 "fee" charges I decided to pay the rest of the account in full (approximately $350).  Then I receive a notice that I have a late charge to pay for this final payment.  This was an outright lie.  I refused to pay it.  Now years later they have tacked on an extra $450 in interest on this ficticious "late fee".  They have called multiple times trying to obtain payment and I've always refused, telling them I don't care if it gets to $4 million and fifty I'm not paying what I don't owe.  Well now in order to get my home refinance with the great interest rates going on now I have to pay them this blackmail fee.  Why are they and other low-life companies like them allowed to continue to do this?  I should also mention a funny related story.  I wanted to open a line of credit at a hardware store so I filled out the required paperwork.  The employee of the store congratulated me as he handed me my new credit account.  When I saw it was Care Credit I almost had a seizure!  I immediately ripped it up and called and canceled it.  Had I realized it was with them I wouldn't have begun the process in the first place.  Wasn't it strange that they would have approved it anyway given the fact that they thought I owed them money from a previous account?  Stay away from Care Credit/GEMB.  You'll be sorry if you don't.