It would be a good idea to at least buy one or several domain .TLDs for open source projects even if you currently aren't planning to make use of them. If nothing else getting a domain prevents someone else from registering it, and you could start using them to make legitimate results higher in search results. This is easiest when you are first starting a project and can check availability before settling on a name.
The problem is that there are hundreds of TLDs so you're not going to be able to register them all. Simplex mentioned they reported the domain. If you do an ICANN lookup you can find an abuse email for the domain, which would be an email for the registrar. You can report to that email and it is likely they will remove the site. You can also go directly to the TLD and report abuse to them as well.
Beyond that search for your project every once and a while and see what comes up or what doesn't come up.