Q: Can I turn off the Personal Hotspot USB default to automatically connect when I plug it in to my computer?

Found a solution.  On your Mac, open System Preferences, open Network, from the list of connections on the left just delete your iPhone USB as a connection by selecting it and clicking the minus button.  This still allows you to connect via bluetooth but does not automatically create a hotspot when plugging in to charge. 

You can also make it inactive, rather than delete it, in that menu if you think you'll ever want to use your iPhone as a wifi hotspot for this mac in the future.

Is there any reason that you would not turn your phone hotspot setting off?

George