You could in theory report it to the DOJ which could in theory ask SE's parent company to register itself as a foreign agent under FARA (and then display prominent notices on its site[s]), if the DOJ were to agree with your claim that (politics)SE is significantly influenced by foreign powers. Good luck with that. (I for one think the DOJ would force Facebook or Twitter to list themselves that way long before they get to politics.SE. After all, those are more notorious aggregators, which contain subsets of foreign-controlled accounts.)
The FARA enforcement unit presently has this contact info
If you believe an individual or entity is violating FARA or has an obligation to register, please contact the FARA Unit:
By E-mail: firstname.lastname@example.org
By Phone: (202) 233-0776 / (202) 233-0777
National Security Division
U.S. Department of Justice
175 N Street, NE
Constitution Square, Building 3 - Room 1.300
Washington, DC 20002
Because investigations are confidential, the FARA Unit cannot provide updates on submissions it has received.
As you found out, the FBI has a Foreign Influence Task Force. I'm not really familiar with it. Its description page says of its goals:
Through the FITF, the FBI is taking a three-pronged approach to this serious threat:
Investigations and operations: The FITF works with FBI field offices across the country to counter the extensive influence operations of our foreign adversaries.
Information and intelligence sharing: The FBI works closely with other intelligence community agencies, as well as with state and local law enforcement partners and election officials, to ensure a common understanding of the threat and a unified strategy to address it.
Private sector partnerships: The FBI considers strategic engagement with U.S. technology companies, including threat indicator sharing, to be important in combating foreign influence actors.
On a subpage they give this contact info:
State and local election officials or campaign staff should report suspicious activity to their local FBI field office and also notify FBI CyWatch at email@example.com or 1-855-292-3937.