A very simple and straightforward way that worked for me is as follows:

1. Stop apache2.

   sudo service apache2 stop
   

2. Uninstall Apache2 and its dependent packages.

   sudo apt-get purge apache2 apache2-utils apache2.2-bin apache2-common
   

   //or 
   sudo apt-get purge apache2 apache2-utils apache2-bin apache2.2-common

3. Use autoremove option to get rid of other dependencies.

   sudo apt-get autoremove
   

4. Check whether there are any configuration files that have not been removed.

   whereis apache2
   

5. If you get a response as follows apache2: /etc/apache2 remove the directory and existing configuration files.

   sudo rm -rf /etc/apache2