A one-liner solution:
sudo sed -i $'s|no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="echo \'Please login as the user \\\\"ubuntu\\\\" rather than the user \\\\"root\\\\".\';echo;sleep 10\" ||g' /root/.ssh/authorized_keys
A one-liner solution:
sudo sed -i $'s|no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="echo \'Please login as the user \\\\"ubuntu\\\\" rather than the user \\\\"root\\\\".\';echo;sleep 10\" ||g' /root/.ssh/authorized_keys