The US government intervened (briefly and ingloriously) in the Russian civil war, alongside the British Empire and France, and I haven't heard of any Americans serving with the White Army outside the bounds of that intervention, so I don't think that's a particularly useful example to use.

The Spanish Civil War is a more interesting case, but the short answer to your question is: no, not to my knowledge.

Several thousand Americans fought with the International Brigades in Spain, but they weren't initially seen as being in violation of US law, and I'm not aware of any prosecutions connected to participation in the war. At the outbreak of the civil war, the 1935 Neutrality Act didn't make participating in other nations' civil wars a crime, a loophole that was partially closed by the subsequent 1937 Neutrality Act.

But FDR, although guilty of a remarkably muddled and indecisive policy approach to the Spanish Civil War, wasn't a simple isolationist: in fact, he became increasingly pro-republican as the war progressed, and there was some tacit US support for the republic. Dominic Tierney has written an entire book on Roosevelt's complex views on Spain: FDR and the Spanish Civil War.

Part of the problem of these kinds of laws was that they were remarkably ineffective: the UK's Foreign Enlistment Act ostensibly did prohibit British citizens from fighting in Spain, but it proved all but unenforceable — here's a good essay on that legal predicament (unfortunately requires Oxford Journals access.)

None of which is to say that the Lincoln Brigade volunteers didn't face problems on their return to the US: they were widely caricatured as communists (which many were) and became targets for persecution during the post-WWII Second Red Scare and the McCarthy era. 'Fought in Spain' became one of the many traits erroneously used as evidence of communist sympathies. Peter Carroll covers that period in extensive detail in The Odyssey of the Abraham Lincoln Brigade.