How to handle rogue customers creating accounts but not buying on my online store?

[Bravos]

Just to set the context: we are definitely non-techie and after our one shop was created it has worked satisfactorily for some time - while we forget everything we learned about creating it in Shopify.  

We now have multiple unknown customers creating accounts but not buying anything, about 2,000 to date and at a rate of tens every day.  This is resulting in complaints from all over the world that we are spamming or have a data breach (not so as we have no idea who these email addreses belong to so they cannot have come from us.) 

We've used the online chat support but do not have the expertise to understand what we are being asked to do. This looks like a Shopify problem to us as this is only possible through the Shopify software of the shop.

How do we escalate this to a responsible human being at Shopify who can deal with it ?

Regards