[Plugin: Participants Database] Field to prevent duplicates how to

Hi. I really like your plug-in it’s very useful.
I have a couple of questions:

1.I don’t have an email field in my database so I need to set another field to prevent duplicates. I read Jacksun post where he says he hack the code to change the “no duplicate check” field. So how do I edit the code?

2. I can’t delete any fields in the Manage Database Fields page. The “X” doesn’t work.

Thank you for your time.

https://www.ads-software.com/extend/plugins/participants-database/