Ah. Thankfully, they all have the same username.

I don't know if there is an instance-neutral way to give directions, but for me I could get a convenient list of all of their usernames as links by using the search function. Then, on desktop, I could open all of them in different tabs (for me it's middle mouse button to open without changing tabs), then put my cursor over 'block user' and use the keyboard shortcut for closing a tab (crtl-w for me) to block them all in under a minute.