How to block subdomain?

I have a project recently I hosted the same on digital ocean but the websites is serving following domains

www.example.com,
host.example.com,
mail.example.com,
webdisk.example.com,
webmail.example.com 

All these sub domains are serving the same content My guess is that serving same content through different domains are not good so I want to block them.

1)How can i block sub domains instead of mail.example.com which is used by postfix.

2)Why there are different sub domains?