Seriously, what can Congress do if someone refuses a subpoena?

[PoliticAverse]

3. Congress has 3 ways to penalize someone for refusing a subpoena...

If you are really interested in this topic I'd recommend reading the following (.pdf):

Congress’s Contempt Power and the Enforcement of Congressional Subpoenas: Law, History, Practice, and Procedure
Todd Garvey
Legislative Attorney
Congressional Research Service
May 12, 2017
(85 pages)

https://fas.org/sgp/crs/misc/RL34097.pdf

Congress has three formal methods by which it can combat non-compliance with a duly issued subpoena. Each of these methods invokes the authority of a separate branch of government. First, the long dormant inherent contempt power permits Congress to rely on its own constitutional authority to detain and imprison a contemnor until the individual complies with congressional demands. Second, the criminal contempt statute permits Congress to certify a contempt citation to the executive branch for the criminal prosecution of the contemnor. Finally, Congress may rely on the judicial branch to enforce a congressional subpoena. Under this procedure, Congress may seek a civil judgmentfrom a federal court declaring that the individual in question is legally obligated to comply with the congressional subpoena.