Huge increase in spam signups, any solution?

Our mailing list manager pointed out there were some suspicious signups, and when I looked into it, there were thousands of bot signups. Many were stuck in Unsubscribed state for “Email Frequency”, but a ton of them were somehow Subscribed.

They have emails like [email protected]. Nonsense.

Many of the ones from the period I wasn’t looking at ended up as Cleaned status, so it seems Mailchimp itself has some recognition that they are bogus, but many others were just sitting there as Subscribed, both causing me to be charged and confusing our internal metrics about the number of actual subscribers.

In our particular case, the form on our site has a radio box for daily/weekly/announcements, and most of these bot signups don’t “fill out” this box, so I can filter the audience to see them, but it seems to me that on sites without this detail, it would be nearly impossible to identify them, beyond just looking at the addresses and deleting the ones that seem suspicious. Many of the signups are from normal looking emails though, I only noticed them because of the missing radio field.

Has anyone else noticed this happening?

Is there anything we can do about it, other than trying to clean them up constantly?

I wonder if the plugin could be updated to include a nonce or some other hidden field that would avoid these bots getting through the form.