How to completely uninstall kubernetes

kubeadm reset 
/*On Debian base Operating systems you can use the following command.*/
# on debian base 
sudo apt-get purge kubeadm kubectl kubelet kubernetes-cni kube* 


/*On CentOs distribution systems you can use the following command.*/
#on centos base
sudo yum remove kubeadm kubectl kubelet kubernetes-cni kube*


# on debian base
sudo apt-get autoremove

#on centos base
sudo yum autoremove

/For all/
sudo rm -rf ~/.kube

use kubeadm reset command. this will un-configure the kubernetes cluster.

If you are clearing the cluster so that you can start again, then, in addition to what @rib47 said, I also do the following to ensure my systems are in a state ready for kubeadm init again:

kubeadm reset -f
rm -rf /etc/cni /etc/kubernetes /var/lib/dockershim /var/lib/etcd /var/lib/kubelet /var/run/kubernetes ~/.kube/*
iptables -F && iptables -X
iptables -t nat -F && iptables -t nat -X
iptables -t raw -F && iptables -t raw -X
iptables -t mangle -F && iptables -t mangle -X
systemctl restart docker

You then need to re-install docker.io, kubeadm, kubectl, and kubelet to make sure they are at the latest versions for your distribution before you re-initialize the cluster.

EDIT: Discovered that calico adds firewall rules to the raw table so that needs clearing out as well.

In my "Ubuntu 16.04", I use next steps to completely remove and clean Kubernetes (installed with "apt-get"):

kubeadm reset
sudo apt-get purge kubeadm kubectl kubelet kubernetes-cni kube*   
sudo apt-get autoremove  
sudo rm -rf ~/.kube

And restart the computer.