If you have installed minikube you could try to disable it in rc.conf

sudo update-rc.d minikube disable 

or

sudo update-rc.d minikube remove

For further detail look at the man page for update-rc.d by typing the command man update-rc.d

Try systemctl disable kubelet.service

I use the same setup on Ubuntu 18.04 and had the same issue.
After starting minikube without virtualization using sudo minikube start --vm-driver=none I can stop it with sudo minikube stop but after reboot the cluster is up again.

For me it was kubelet running on my machine that would start the cluster on reboot. systemctl disable kubelet.service fixed the issue.

You can check if kubelet is enabled on your machine by running systemctl is-active kubelet