My ex is a computer "expert". Is he using administrative privileges to remotely control/limit use of my computer? My Ex knows a lot about computers - I know nothing about the hardware and infrastructure... 
I bought my own personal MacBookAir for my private use only. I'm not able to change some settings or be the "administrator". Here are system preferences snapshots under Security and Privacy. It seems to look like I'm on a network under "WINS" (?) and I can't change anything in "Proxies" or "802.1X" 

I turned on FireVault (with a recovery key) and FireWall. I also set a login password, required immediately, and only allow apps from the Apple Store. 
The other pics are my WiFi setting and Directory Utilities if it's helpful. I have Komros installed and Avast and use them every time I log-on. 
Thank you!
If you are really worried, backup and then do a wipe and re-install. This will get rid of most unwanted things. Then manually restore important documents and files from backup. If you are not THAT worried, install a firewall/traffic monitor (LuLu, LittleSnitch, RadioSilence) and just monitor the network activities.

The reason you can't "Administrate" can just be that you were not added as an "Administrator" when the account was created. Check in System Preferences > Users &amp; Groups to see if you are an administrator. You can add yourself (if you are not already) as a sudoer and make yourself an administrator though I'm don't feel comfortable explaining this process, use your search engine of choice.

You can list hidden users, check running processes and network traffic in search of something suspicious though it is not always trivial and if you are really that worried, wipe and re-install.