It is a bit of a tough one.
From your perspective you paid the bill, and thought everything was ok. You knew you had an extra device, but you weren't using it anymore, so why would you need to do anything about it.
From Cisco Meraki's side they were basically doing debt collection. They had a customer that had part paid their bill (from their perspective). They would have sent a number of reminders. There would have been a big banner in the dashboard. I'm guessing you never contacted Cisco Meraki about any of the warnings otherwise I know they would have helped with this situation. What can you do as a business with a non-communicative customer who has only paid part of their bill? Eventually you have no choice but to cut them loose. I've done it before. Some customers are simply not worth having.
Ideally their would have been a Cisco Meraki partner involved to help - the original Cisco partner that sold you the solution and designed and deployed it. As a partner I can tell you I follow up every single warning we get from Cisco Meraki about our customers to make sure no one has to suffer a horrible fate. It is usually such a trivial matter to do.
Perhaps you just got your kit from a box shifter, and did most of the work yourself, I don't know. I guess the more of the task you take on yourself the more responsible you are for the situation.
I'm really sorry that this had to happen to you. It is a pretty terrible position to be put in. I don't really see what Cisco Meraki could have done differently to be honest.
What do you do in your company when customers don'y fully pay their bill and they don't respond to you when you try and contact them about it?