Any question answered...

Is there an irreversible and permanent way of removing all internet capabilities from a macbook? It's a 2012 ish white one

Do you still need the macbook to function otherwise? 😁

Seriously though... are you a systems engineer? If yes, then no. If no, then yes.

I can basically break any ability for it to do anything even if you connected it to any network. The macbook would no longer be able to receive any updates or interact even with the local network, but presumably you're good with all of this and are OK using USB sticks to move files on and off it.

The phrase you want to be Googling is "air gap"... and air gapped machines are physically incapable of connecting to any network. In security this is the standard for working with malware and viruses of a certain class to observe them.

Here's the kind of thing we're talking about: https://tech.firstlook.media/air-gapping-part-one

And basically you take your Macbook to a repair shop and ask them to remove the network card and accept the risk of doing so (may kill the machine if the engineer is an idiot).

Afterwards you have a Macbook that can do everything except communicate with anything else.