Default Apache page shows up on localhost:80 even after "stopping" apache2 service

you can double check that there are no process still running by do something like this: -

ps aux | grep apache

if apache2 is still running it will display results

kill them using

sudo killall apache2

You will need to determine why the process went rouge from the init control. this can happen if you are running a very busy apache server, it will indicate that there is some performance related issue but the reason for this are many.

check that there are no other web applications running that might be starting apache2 or even a mod version httpd manually with a /usr/sbin/apache2 -d statment. I doubt it though as you would more likely incur port locking issues especially if both using standard ports

of course what are the apache logs saying?