You can make use of a custom DNS server that blocks connection to most of the Nintendo Servers in order to block updates from downloading and installing.
Blocking access to Nintendo Servers also blocks access to the Nintendo eShop, online gaming and game updates.
- Start your Nintendo Switch normally (not in Custom Firmware mode)
- Open Settings and navigate to Internet
- Select your WiFi network (with the green check mark)
- Click Change Settings
- Click next until you see DNS Settings
- Set Primary DNS to 126.96.36.199
- Set Secondary DNS to 188.8.131.52
- Save settings