Now, if you believe that these agencies have amassed and hoarded enough of the money they've already been given that they now have an endowment that can perpetually fund their activities, then yes, that would mean you really can't stop them. But I'd find it hard to believe that's the case; what they do is not cheap, and I'd be surprised if there's all that much left over after they spend their yearly budget on regular operations.
If Congress voted to completely defund these agencies, they'd go away. Not immediately, and I'm sure there will still be some in power who create limited shadow agencies that run on much, much smaller budgets, but they can't operate at the scale they do now without continued funding.
Really, though, I'd expect them to maintain Congress' support through non-lethal means. Bribery, blackmail, stuff like that. A Congressperson in your pocket is much more useful than a suspiciously-dead body.
But still, if US citizens as a whole really wanted them gone, they'd just continue voting in people who will take a hard line against funding them.
I don't expect something like this to come to pass, mind you, but it's possible.
What do you think In-Q-Tel[1] is for? It's about ensuring that the TLAs will exist in perpetuity regardless of Congress.
And the hope, with all this surrounding us, that somebody must made exclusive findings ? P-: