(I'll start with the case of George Santos because that limits it to a specific office and a specific accusation of misbehaving. I'll deal with other offices later but those are mostly kept brief because a lot would depends on the circumstances)
The House can expel him as a member. That's the path House Speaker McCarthy mentioned to journalists in late January. According to NBC News quoting Speaker McCarthy:
“If for some way when we go through Ethics that he has broken the law, then we will remove him, but it’s not my role,” McCarthy said. “I believe in the rule of law. A person’s innocent until proven guilty.”
So how does that work and who is we? If we look at the US Constitution Annotated we'll find that Article I, Section 5, Clause 2 reads:
Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behaviour, and, with the Concurrence of two thirds, expel a Member.
Indeed, there is precedent for removing members even though the cause for expelling has quite a high bar historically. The House website lists 5 members who have been expelled and the reasons included either disloyalty to the Union or bribery related convictions in each case.
I think to keep this focused, let's keep this about federal offices, like Congress and the presidency.
My question is prompted by the actions of George Santos, but I'm really interested in what can be done, is being done, or is already in place to prevent this from happening (again).
To answer your question, there is probably not a lot that can be done before someone gets elected to a political office other than getting someone else elected. When someone does get elected to Congress then they are subject to the respective ethics committees.
As for the executive, there is oversight through the Attorney General (who may appoint a special counsel) as well as the Office of the Inspector General.
Most of the courts have ethics committees too, you could find those by searching for the name of the court and the term 'judicial ethics committee'.
The notable exception to this is the United States Supreme Court in which Justices seem to be mostly self-governing. According to a 2022 publication in the Atlantic:
Chief Justice Roberts has noted that Supreme Court justices voluntarily consult the Code of Conduct and other ethical rules for guidance. He has also pointed out that the justices can seek ethical advice from a variety of sources, including the Court’s Legal Office, the Judicial Conference’s Committee on Codes of Conduct, and their colleagues. But this is voluntary, and each justice decides independently whether and how ethical rules apply in any particular case. No one—including the chief justice—has the ability to alter a justice’s self-judgment.