How to block a contact from mailing but keep them on your database to prevent future sign ups

We have some competitors who have signed up to our website and subsequently created as a contact on Infusionsoft. What is the best way to block them from future mailings - as I assume it is best to keep them on your database in case they simply re-sign up.

Under Admin->settings click ‘application’ and to the right you have a spam filter option (see image)

Thank you - just looked that up and sounds like a good approach!