Anyone is able to subscribe to your list, even bots (most notably known as spambots). At the moment, there are no mechanisms that can block a user from subscribing.
However, there are a couple of workarounds that automatically unsubscribe an unwanted subscriber, or block them from receiving newsletters.
There’s a workaround that you can implement through the use of an automation that would immediately unsubscribe the user from your list as soon as they sign up.
To start building this automation:
Head to Automations.
Select Create workflow.
Name the workflow.
For the workflow trigger, select Updated field.
For the custom field section, select Email, then is equal, and finally, enter the unwanted subscriber’s email address.
Click on the plus sign (+) below the trigger block and select Action.
Choose the action Mark as unsubscribed.
Turn the automation on.
And that’s it! Now every time the unwanted subscriber tries to resubscribe, they will automatically be unsubscribed and you shouldn’t have to worry about them anymore.
This sort of blacklist allows you to exclude certain subscribers from receiving your newsletters and automation emails by creating a segment with the filter:
Fields > Email > Equals > [email address you don’t want to send newsletters to]
Head to Subscribers.
Click on Choose a filter and select Fields.
In the next fields, select Email followed by Equals.
In the last field, enter the email address you don’t want to send newsletters to.
Click Save as segment, give it a name, and click Create.
Add the email addresses of all the unwanted subscribers to this segment so you can exclude them from receiving your emails.
To add additional email addresses to the segment, select + Add another set of conditions and repeat steps 2-4.
To exclude the subscribers in this blacklist from receiving any newsletters or automations, click on Exclude segment on the Recipients page of a campaign. The campaign will not be sent to those who are in this segment.
Read How to exclude subscribers from a campaign or an automation for more information.