Hi Supporter,
1. The string on the discuss app story form button uses the translation from this language constant: APP_USER_EASYDISCUSS_STORY from JoomlaFolder/administrator/language/en-GB/en-GB.plg_app_user_discuss.ini

2. The 'New Discussion' option under the 'Stream' tab in the Discussion app actually determines whether stream items regarding newly created discussion should be shown in the stream, not the visibility of the discussion button in the story panel.
Unfortunately, there is no setting to turn off the button without disabling the app.