First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac. This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.
There are no known viruses, i.e. self propagating, for Macs. There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.
Anti Virus developers try to group all types as viruses into their ad campaigns of fear. They do a poor job of the detecting and isolating the adware and malware. Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.
There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it. The free version is more than adequate for most users.
Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint. Read these two articles: Public VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu). Additionally, a new study ("Apple Offers Apps With Ties to Chinese Military”) is specifically about VPN apps in Apple’s App Store.
After removing Norton software according to the developer's instructions yYou can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name. For the Norton files you'd do the following search(es):
1 - Name contains norton
2 - Name contains AV
3 - Name contains virus
Any files that are found to be associated with Norton A/V can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.
FAF can search areas that Spotlight can't like invisible folders, system folders and packages.
If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to How to use safe mode on your Mac, run Find Any File again and delete from there.
Note: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.
