samsung - Used note 4 clean spyware

I purchased a used Note 4 off ebay and want to make completely sure there are no spy apps or malware on it. I know some spy apps can be installed into the system partition and survive a factory reset. I discovered an app called cerberus that could do this and could be used to spy on me and blackmail me. How do I completely clean my phone of all spy apps or malware and completely wipe everything including the system partition erasing everything.