The short answer to this question is that you can't.
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.
Alternately you can choose to block the participant/user. Read the article Blocking Participants to understand how this can be done.
If this is required for legal purposes you can reach out to our support team via chat or email firstname.lastname@example.org.