Generally PACs don't directly hand much money to politicians or their campaigns. Instead, they make and run ads, they organize speaking events, they spend money instead of some politician's campaign. That's why all the political ads have some "This ad was paid for by Americans for Extreme Goodliness" taglines. They're not being run by the campaign itself (which has limits on its funding, mostly transparency stuff, but not necessarily), they're run by PACs.

And that's basically impossible to curtail, without seriously violating fundamental 1st amendment protections.