It is probably some sort of bot, or an attempt to find a vulnerability in the website (wouldn’t worry too much, Shopify has pretty good security systems in place). It shouldn’t have a detrimental effect on your store beyond being very annoying.
You can delete their user account from your Shopify admin. That should help**,** although in theory they can just create another.
Go to Customers> Click On Problem Customer> More Actions> Delete Customer
There are apps that claim to solve spam accounts, but not sure how effective they are. May be worth looking into.
Hope that helps!
@biznazz101