The first thing you have to do to prevent someone from taking over your domain is to block it for transfer. If yours is a generic domain, don't worry, at dinahosting we already block it for you by default as soon as you register it. You can check it in your Control Panel, Domains section. Next to the Authcode you have the Unlock option, so unless you do it yourself, the domain will be protected.
In the event that someone has usurped your identity or registered a domain name associated with your brand in bad faith, it is advisable that you discuss it with a specialist in Internet legal matters for advice. If you want, take a look at our Legal Advice service.