Any question answered...

Is there an irreversible and permanent way of removing all internet capabilities from a macbook?

If you really want to kill the ability to read or write to a network, you would need to physically destroy all means of external I/O, such as Ethernet adapters (wired and wireless), USB adapters, any other peripheral ports. That's going to involve chopping out sections of the motherboard if you want to make it truly irreversible.

You can cut the WiFi and Bluetooth antenna cables easily enough. Most of them have it on a separate module you can take out.

Other ports are much harder.

1. Set up a user account with no internet access or the ability to add such access.
   
2. If it's for you, do this while incredibly drunk and you are sure to forget the admin password.
   
3. Profit
   

It's not irreversible but get a friend to set the admin password to something you don't know and use that account to set dummy IP addresses for all the interfaces.

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.