What stops the mega wealthy is competition. What prevents competition is the government. As long as you have freedom of association, you can choose to associate with companies that don't exploit you. There is room for basic labor laws preventing child labor, discrimination, and a few other things.
OSHA should be privatized and made to be a safety standards company similar to how Underwriters Laboratories and other safety industries work. OSHA rules are out of date and often absurd.
The EPA should also be privatized. Let the public see what the environmental rules are and boycott companies that destroy the environment. The EPA as is currently set up is accountable to no one and can effectively ban anything based on if they determine it is a pollutant.
The FCC should regulate what frequencies can be used and regulate interference so jamming is considered the same as blocking speech.
Libertarians don't believe in NO government or a ridiculously tiny government. They believe in a limited government (limited by a document, like say the Constitution), an accountable government (fair elections, unlike what we have now), reasonable and low taxes, and personal freedom. History has shown that free people do a very good (but not perfect job) of advancing society and creating wealth for the entire population even if it is unevenly distributed.
Most libertarians believe the government should do a whole lot for society, but at the smallest level of government. Fire departments, police, schools, etc should be manages by the local government, not the feds. Federal officials aren't accountable over local concerns.
Your view is that we need to keep money out of politics. I see that as dealing with the symptom, not the problem. I want to get politics out of money. The problem is that the government is so large and powerful that they can control large amounts of private sector money. Take away their power, contracts, tax benefits, and straight up handouts and then there isn't an incentive to buy politicians anymore. Why spend a billion dollars to win a position that won't return a billion dollars in "influence"?