Discuss is a button as part of Sidebar. Therefore not part of UI Action

To deactivate Sidebar:

Change application to: Omni-Experience Standard Feature Set

1. Go to 'Settings' under 'Conversational Interfaces' in left navigator

2. Select 'Sidebar' from left pane

3. Deactivate in toggle button

4. Click 'Save'

If need to disable to specific table, create a record under "Custom Selection" same page as step 2 above and list the tables you want to still contain this discuss button.