How to hide X-Powered-By and Server headers
First enable the mod_headers module:
sudo a2enmod headers
Then add this to your apache2.conf:
# Hide X-Powered-By and Server headers
Header always unset "X-Powered-By"
ServerTokens Prod
ServerSignature Off
Now restart Apache:
/etc/init.d/apache2 force-reload
This is security through obscurity at it's finest...