Another domain points to my webserver
Solution 1:
You can easily prevent this at the webserver level by explicitly defining your ServerName directives. Non-matching hosts would simply be funneled to the default page (or whatever you choose).
It is possible to fool the Host: header, but this prevents the most common ways. More damaging is that the suspicious domain will duplicate content on your domain which hurts SEO ratings (search engines hate duplicate websites), so the Apache fix would do the trick.