Https listens at port 443 usually, and regular http listens at 80.
You have a few choices. To be drastic, you can configure apache to only listen to port 443. Then if somebody tries http://yourdomain.com
, they would get host not found, or some such error. A more friendly solution would be to create a page at http that forces a redirect to https, but that wouldn't be absolute. People could still browse to pages other than the first page under http.