A very simple and straightforward way that worked for me is as follows:
Stop apache2.
sudo service apache2 stop
Uninstall Apache2 and its dependent packages.
sudo apt-get purge apache2 apache2-utils apache2.2-bin apache2-common
Use autoremove option to get rid of other dependencies.
sudo apt-get autoremove
Check whether there are any configuration files that have not been removed.
whereis apache2
If you get a response as follows apache2: /etc/apache2 remove the directory and existing configuration files.
sudo rm -rf /etc/apache2
Source: How to uninstall and remove Apache2 on Ubuntu or Debian by Dan Nanni
https://installlion.com/ubuntu/xenial/main/a/apache2/uninstall/index.html