How can I irreversibly prevent Safari from being used for web browsing?
Enable parental controls in System Preferences and black list Safari on Mojave and older. On Catalina it’s called screen time, but you can blocks apps like a safari there as well. 

If you need the high security option, get the self control app. Warning, you will not be able to use the internet or blocked sites until the time you enter passes. If you mistakenly choose one day rather than one hour (or worse), you will have to erase your Mac or wait for the time to expire.


https://selfcontrolapp.com