It's even more easier to do it if you have modular smartphone like Fairphone, they can be totally disassembled to parts, including antenna.
But if you can't do it you can destroy/damage the SIM card slot in order to avoid someone from your home or a burglar to use it. And if you can't easily access it on your phone, you can just put some glue into it.
That might not be enough with new smartphones, because most of them have eSIM that can be downloaded and installed, so there is no physical card slow to glue.
Even worse news are coming from Apple who are planning to fully remove card slot and use only eSIM in future.
If Apple really starts doing it, than you know most other brands will follow like always.
Maybe so, but there is also WiFi, Bluetooth, RFID, NFC, and other connectivity modules to consider too. Disabling all of them without breaking the phone is difficult. It is far easier to start with an old laptop which does not have the majority of these at all and then just remove the WiFi card.
Most of this can easily be disabled with Airplane mode, so it's not that hard, unless you want to permanently remove them.
Laptop is better for many things, but you can't disable or remove anything unless you have specific models
(example old Thinkpads), most laptops today have everything soldered on board.