Blackmailed by an essay writing service. What should I do?

Hey, I really need some advice because I'm starting to panic. I used a website to help with my assignments while my son was sick. At first, they did okay, though there were a few delays. I didn't mind that I only got a passing grade instead of the distinction I paid for and didn't make a fuss about it.
After that experience, I decided to use them for two more assignments. By then, I found out I had failed one of the previous ones, which I complained about since they advertised a money-back guarantee.

I paid for the next two assignments, and they promised to deliver them on time. I ended up not using the new ones because one was poorly done and the other never arrived. But I did use their previous work.

I requested a refund, and they asked for my bank details to process it. Later, I checked in to see if the refund had been issued and got a strange reply saying they had entered all my information into a "Global database" and that their compliance team would contact my university for further verification, taking 4 to 5 business days.
When I questioned why they needed to involve my university, they said it was to remove my name from this database and demanded an additional $300.

I'm not worried about the new assignments since I did them myself and didn't use their work, so there’s nothing they can hold against me there. However, I did use their earlier assignments.

I begged them not to contact my university and not to put me on any database. I realize now it was a mistake to use their services as it was my first time doing something like this.

What should I do next? Should I just keep quiet and stop contacting them? Can they actually contact my university?
They absolutely won't give you a refund, no matter what they promise. All that bullshit about contacting your school is just to scare you off from asking for your money back. If you haven't told them the name of your university, you should just call your bank, show them the company's guarantee and the emails they sent you when you asked for a refund.

If you've already shared your uni name, it's best to just accept it as a lesson learned and not provoke them into actually sending your old papers to your school out of spite. It's best to ignore their emails.
Just ignore their threats. They’ll give up and find someone else to bother if you don’t engage with them. FYI, there’s no such thing as a global database for academic papers. Some scammers will try to convince you there is one, but when you push them for details, they might say it’s part of their own plagiarism detection system.

Also, a quick piece of advice about hiring company writers: I wouldn’t recommend doing it. But if you do go down that route, try not to use the same company more than twice. In my experience, the quality of the work drops after a couple of papers.