Remove snaps:

  sudo apt-get remove snapd
  sudo apt-get remove lxd-agent-loader # 20.04

The key to keeping them uninstalled is to pin the packages so they don't get "upgraded," which results in them being reinstalled.

  sudo apt-mark hold snapd
  sudo apt-mark hold lxd-agent-loader # 20.04

If you want to go the next step in cleanup:

  sudo umount /var/lib/snapd/snaps/*.snap
  sudo rm -rvf /var/lib/snapd/*
  sudo rm -vf /etc/systemd/system/snap-*.mount
  sudo rm -vf /etc/systemd/system/snap-*.service
  sudo rm -vf /etc/systemd/system/multi-user.target.wants/snap-*.mount
  sudo rm -vrf /snap/*