Re: I want to completely forbid changing the “display_name”
The _CK_ plugin is nearly what I needed because it disable display_name for all user, also for admins and moderator.
But with my previous plugin to show display_names in bb_press 0.9.x and this new plugin to disable them in bb_press 1.x , I could make my own version of plugin 
I just proposed it in the official bbpress.org/plugins list, but I don’t know how this “requests” works. I guess I have to wait some validation/moderation.
Thank you all for your answers
