There are a few reasons why you as the admin would want to block a participant's access to EHQ. Arguably, most likely because a user has repeatedly breached your site's rule and has therefore lost their rights to continue using your site.
As a site admin you can at any time block and unblock participants. This can be done by clicking on the Team link in the side navigation bar and then clicking on the Block option in the Team page.
Being blocked results in the participant not being able to make any submissions where verified participation is required or access protected projects they had been granted access to. In addition, it will remove them from the mailing list of your EHQ site and they would no longer receive any newsletter you sent to your database from within EHQ.
It should be noted that project admins CANNOT block participants. In addition, participants are not automatically notified about their status. If you wanted to let the user know that their access has been removed, you would need to contact them separately.
Note: In EHQ you cannot delete participants completely. Read the article Can I permanently delete a participant? to understand the reasons.