[Plugin: Participants Database] Don't allow registration

In my last post I didn’t explain quite well what I want it.

We are giving away somo prizes and we want to keep a record of the people who already claim their reward. So I need to have a single record per person not update it.

With the post from Jacksum the database is alreday updated with a costum field. Now I need to display a “This ticket number is already registered” alert instead of just update the data.

Is this possible? I’m a coding newbie so I’m pretty helpless.
Thank you.

http://ww.wp.xz.cn/extend/plugins/participants-database/