In recent versions of macOS with the Server.app, the Server software has been more and more persistent about having Apache running. Even if you’re not using any of the web services, it still likes to have Apache running as a proxy service.
On older versions of Server, you could type in:
sudo launchctl unload -w /System/Library/LaunchDaemons/org.apache.httpd.plist
On newer versions of Server, you also have to stop the Service Proxy service, which uses apache.
sudo launchctl unload -w /Applications/Server.app/Contents/ServerRoot/System/Library/LaunchDaemons/com.apple.serviceproxy.plist
If you enter the first launchctl command and get an error that looks something like:
/System/Library/LaunchDaemons/org.apache.httpd.plist: Could not find specified service
Then, you need to use the second launchctl command to also unload the instance of Apache started by the Service Proxy service.