How do I protect random people pointing their domain to my server?

I just found out that flashsale.info is pointing to my server. How do I stop this from happening? Is there anything I can do?

There is nothing you can do to forcibly stop other people from pointing their domains at your server. It may not be malicious; it may be that they have simply made a typo in their zone file, or that they have forgotten to delete an old entry. So the first thing to do is to simply contact them and politely inform them of the error.

If they don't respond appropriately, the simplest solution is to create a virtual host or a redirect rule that will simply respond with an error message to anyone requesting the wrong hostname.

Since you haven't specified what web server software you're using, I can't help with any configuration options. If it's apache, you should start by reading this explanation of how apache handles hostnames and the canonical answer about how rewrites work in apache.