Hi,
You can prevent this by adding more security to your website. You can start to add a captcha to your subscription forms, but you then would need to use the Essential or Enterprise version. (https://docs.acymailing.com/setup/configuration/security)
A second line of defense would be to add Acychecker to your site. It helps you to keep your subscribers list healthy. you can read more about this tool here: https://www.acychecker.com/
There are numereous tools available that can help you to protect your site against attacks and spam submissions in your forms. You can find them in the Joomla Extensions Directory or Wordpress Plugin Directory.
