'Mailchimp didn't save your recent Form Block submission'

Site URL: http://www.arianamartin.com

I have a newsletter sign up form on my site linked with Mailchimp. I'm getting LOTS of emails from Squarespace titled 'Mailchimp didn't save your recent Form Block submission' due to spam/suspicious activity.

They are all clearly phishing attempts from spambots, because the names/email addresses used are suspicious (see screenshot). I used to get maybe one notification like this per week, but am now getting several per day.

Does anyone know why this is happening and how to make my form more secure?

Thanks in advance!