Enable wildcard domains while disabling subdomains

I have a one-click WP install strictly for parking domains for sale using a highly regarded plugin/theme that will display an offer form for any domain pointing at the server provided there is a DNS entry.

If the server block specifies "server_name _;" it works. However, you can create a subdomain just by visiting it such as pigglywiggly.example.com/

Is there a way to allow the primary domains by wildcard but not allow subdomains? I became suspicious when I saw eliminate-bacterial-vaginosis.example.com in my stats.

Thanks in advance for any solutions or suggestions.