Contact the registrar that the domain is registered with.
You can use any WHOIS search engine to gain the abuse contact details for the registrar, as well as (providing that they haven't made the WHOIS data private) the owner's contact details. I often use the who.is search for WHOIS information (I find that there are a few 'WHOIS search' providers out there who are actually registrars).
The section you'll be interested in is this:
Domain Name: EXAMPLE.COM
Registry Domain ID: 000000_DOMAIN_COM-VRSN
Registrar WHOIS Server: whois.register.it
Registrar URL: http://we.register.it
Updated Date: 2015-08-20T00:00:00Z
Creation Date: 2015-05-20T00:00:00Z
Registrar Registration Expiration Date: 2016-05-20T00:00:00Z
Registrar: REGISTER.IT S.P.A.
Registrar IANA ID: 168
Registrar Abuse Contact Email: abuse[at]register.it
Registrar Abuse Contact Phone: +39.0353230310
Obviously the registrar's information will vary from domain to domain, but you'll still be wanting to know this info:
Registrar Registration Expiration Date - If the domain is expiring inside a month, I personally wouldn't bother reporting it unless you feel it is doing you serious harm or that the domain's expiration date is extended (the owner will probably get notifications that it's expiring a couple of weeks prior)
Registrar Abuse Contact Email/Phone- Here's where to contact to have the domain investigated for abuse (this includes purporting to be another website, which is the case here). If successful, the domain will be suspended.
You will also be interested in the
Registry Tech ID: section of the WHOIS record, which lists additional details for the registrar. Depending on whether the registrar is a reseller, this information could be different to the information mentioned above.
If you still can't get hold of the registrar, contact ICANN directly (if it's a domain regulated by them), or the appropriate regulator for the TLD or SLD of the offending website. If the registrar won't do anything about it/can't contact they the regulator will.