Congress can't simply pass a law to get around this decision. Since the basis of the decision was the First Amendment, Congress can't pass any law that is not compatible with the decision. While the Supreme Court decided that corporations have rights under the First Amendment many decades ago, this decision seems to carry it to an extreme as far as Congressional ability to regulate that speech. Personally, I feel I am quite capable of making a distinction between the chilling effects of suppressing individual speech and the effects of limiting corporate speech. Justice Stevens wrote a great dissenting opinion. Here are a couple of my favorite lines: "Under the majority’s view, I suppose it may be a First Amendment problem that corporations are not permitted to vote, given that voting is, among other things, a form of free speech."

And:

"While American democracy is imperfect, few outside the majority of this Court would have thought its flaws included a dearth of corporate money in politics."

Congress can't pass a law that is antithetical to the holding in Citizens United. The Court took the case as an as applied challenge, but their holding (that restrictions on corporate advertising and express speech, for political candidates and issues, are unconstitutional) applies to all future Congressional action. Congress can try to pass a law that runs afoul of the holding, but if they choose to do so, and if the Executive Branch chooses to enforce it, a ruling from a district court will be sufficient to invalidate the law.
As Justice Kennedy states in his majority opinion:
"we return tot he principle established . . . that the Government may not suppress political speech on the basis of the speaker's corporate identity." In its holding, the Court overturned a prior ruling in the Austin case which allowed for restrictions on corporate spending on express political activities. Congress is acutely aware of the ramifications of Supreme Court opinions and rarely, if ever, do they intentionally thumb their nose at the Court and pass a law that is directly contrary to the Court's holdings.