What if you just don't connect it to any network, ever?
How do you stop it from connecting? These days most laptops, at least, have WiFi, Bluetooth, BLE (really distinct from classic buetooth), and maybe other radio-networking capabilities (GSM, LTE, ZigBee, 6LoWPAN, 6LoWPAN-over-Bluettoth-4.2) built-in. Also infrared and ultrasonic-capable audio interfaces with microphones and speakers. Even with the ones that DO have a switch to turn the radios off the switch normally just tells the software not to talk on the radio - which the software is free to ignore.
(Not to mention that the remote-administration hardware/firmware built into the chips by the major manufacturers can, and does, listen on the radios these days for remote-administration commands, comes in UNDER the OS, and can't be disabled.)
Then there's the question of what good the computer is to you if it's NOT connected to a network?