"Proof" when it comes to disproving the existence of a modern, godlike Illuminati comes from the lack of hard evidence for its existence.
With so many versions of what the Illuminati could be (Satanic? Masonic? Atheist? Reptilian?), theorists can't even agree on who or what they are. And, all of those artists who put occult or Illuminati symbolism in their videos? They're doing it on purpose, either because they know you'll talk about it, or because their production team couldn't think of any other designs to use. If this organization wanted to rule the world in secret, they're doing a shitty job. First rule of Fight Club is: You don't talk about Fight Club. Each and every one one of those celebrities would have been kicked out by now for blatantly flaunting their membership. Also, those YouTube videos and websites exposing the Illuminati would likely have been mysteriously erased by now if the group had complete control over things like the Internet.
Going back to the purpose of conspiracy theories in general, they're meant to make sense of the impossible (even if the theory itself doesn't make sense when you dig deeper). Some would like to think someone like Jay Z can become successful on his own, not with the help of a centuries-old secret society.
If there are Illuminati, conspiracy theorists probably aren't going to be very happy when they see how much power the group doesn't have. And, if they do exist, they may be completely different than what the conspiracy theorists have cooked up. Secret societies have been around for ages, but for one with the power to influence world events and shape history to its will? That would be extraordinary. But, it's hard to definitively say that they don't exist. In fact, you probably can't. So the only "proof" you're going to get is the kind that debunks single theories (like Reptilians), and not the existence of the group as a whole. It's an unfalsifiable theory.
And no, Tupac wasn't a member.