You can't.
Even if your domain name is your own legal name or your company name, anyone can buy any name that is not already owned. Of course, if someone buys another version of your domain name (same name, but with a different extension), and then publishes a site that is designed to hurt you or your enterprise, you may have legal recourse. Where you live and work, where the attacker lives and works, and the nature of the attack will all influence what you can do, if anything at all.
But even if you are fortunate enough to actually have recourse, the far better (and cheaper) option is to simply buy as many versions of your domain name as you can justify financially. Then pick the main one, host your site at that address, and have all the others automatically forward traffic to that same main address.