Once you've published a question, you can still remove a reason added by you or a participant.
Go in your administrator page, and click on the question whose you want to remove the reason.
To remove the reason, change its status from 'active' to 'disabled'.
To review reasons before they appear live on the site, click on the 'pre-moderation' option in your question. Then when a participant add their own reason, you'll receive an email notification. The reason status will be 'pending' until you decide to make it active it or to leave it disabled.