Hi and welcome to the forums 
As far as I’m aware you need to report them through Funcom’s helpdesk - Contact Us — Conan Exiles Help Center
I suggest having a read of the Official Server Guidelines (because that will help you to find what sort of evidence to include) and you might also want to take a look at the post about Land Claim Abuse to get some examples of the sorts of bases Funcom is mostly looking for.
I hope that helps