The first step is to hide the Edit button. Go to Options* > Restrictions. Toggle Edit Mode to OFF.
If you just want to set a password on the Options button, go back to Options > Lock Options. Toggle Lock to ON. Enter and confirm a password, then tap Save.
To hide the Options button, go into the Settings app. Scroll down the list until you get to your apps. Tap the Proloquo2Go section. Toggle Show options button to OFF.
Then, when you want to edit again, go back into the Settings app and re-enable the Show options button setting. Then, open Proloquo2Go, go to Options > Restrictions, and toggle Edit Mode back to ON/green.
*On iPhone and iPod touch, you will find the Options button in the Tools popup at the far left of the bottom toolbar.