Let's be realistic about this. It is almost impossible to legally revoke someone's citizenship and for good reason. The last thing we want is a government who can willy-nilly start revoking citizenship of political opponents.
The only laws pertaining to revoking of the citizenship of a naturalized citizen (which Soros is) are:
1. Lying on citizenship application (and we would be far out of the statute of limitations to prosecute for that unless there is a war crime, espionage, or murder charge which doesn't have the same statue).
2. Refusal to testify before Congress, specifically for issues related to subversive acts. This could potentially be used, but all Soros would need to do is show up and take the 5th and walk out.
3. Membership in subversive groups specifically identified in current statute. Several terrorists organizations are part of this. It also would cover Nazi party affiliation (the original, not current groups using the same name). Before anyone puts a bulls eye on this, it has been discussed with Soros before and it doesn't apply in his case because he was a minor.
4. Dishonorable Military Discharge. Not applicable here. It would only apply to those who where naturalized while serving.
In all these cases, it requires a very high threshold of conviction and has an appeals process. Considering Soros' wealth (ie, buy the best lawyers) and the state of the judicial system (his home district where there is a chance he could be prosecuted is very liberal), the chance is slim to none.
What it could do, however, is set precedent for going after other political opponents in the future. It could create a system where this is common practice.
It all comes down to the question, what are you willing to give up in long term liberty for short term small victories?