Hey there,
In order to disable this plugin, you can go to Plugin Manager to disable the smart search plugin that you don't want users to find it in the search. (Kindly refer 1st screenshot)
Example: In order to remove 'Article' in the search, you need to disable Smart Search - Content plugin.
Do take note that the 'Select Type' you need to choose 'finder' in order to look for it.
Once you finished disabling the plugin that you don't want, go to Smart Search Manager and click 'Clear Index' then click 'Index'. (Kindly refer 2nd screenshot)
Can you give it a check whether these articles still searchable?