Yes, Citizens United made it explicit that money is speech and as such can't be hindered. I suspect that because of this ruling, we couldn't just pass a law that prohibits money in politics because the ruling precludes it.

A constitutional amendment would nuke the current limitations, but it's impossible in practice for us to get there. On the other hand, as we've seen as of late, no decision by the Supreme Court is set in stone, so we could, in theory, get Citizens rolled back and then some. That also seems unlikely in the short term.