Prevent the upgrade from ubuntu 16.04 to 18.04

So, new here then …

I demoted my Ubuntu disk from 18.04 to 16.04 (for tensorflow 1.12 and cuda 9. I had to downgrade.) My question is how to keep it at 16 so you can use sudo apt-get upgrade & # 39; # 39; safely do not break the whole system …?