I am very sorry to hear this issue is happening. We definitely want to understand what is going on.
Is the sign-up form on your website from Constant Contact or integrated through a third-party system? If it is from a third-party system does it give you an option to set up Captcha? That might be an effective way to curb the number of spam sign-ups.
If you do not feel comfortable posting publicly you can also reach us at social_support(at)constantcontact(dot)com with your username, a list of the spam contacts if possible, and a reference to this post and we can investigate further for you.
Nick S. Social Media & Content Manager
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