There a few reasons why you as the admin would want to block a participant's access to your 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 simply clicking on 'block' in your People tab.
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 can NOT 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.
Can I delete participants off my database altogether?
In short: No.
This has several reasons, some of the more important ones are:
a) by deleting a participant, you would also delete all of his/her submissions without any record of those ever being received;
b) once removed, a participant would be able to register again with the same email address but potentially a different screen name. This is particularly problematic if you want to remove someone from your database because they have been repeatedly breaching the site's rules; and
c) if you were to delete all records of a participant, the submissions of that particular user, e.g. forum comments, would be irreversibly removed and with them all replies that this comment received from other participants. In other words, it would render the forum discussion useless.