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What about just turning off your network?

What about programs that run on your computer so you don't even need the Internet for them?

OP isn't worried about it not working without network, he is worried about it sneakily sending his data somewhere.

That means you have to keep the network off for as long as you’re using the app, which is inconvenient.

In chrome you can turn off the network per tab.

In Firefox I use this extension for the same purpose

This only works on Google's OS, Chrome.