Somehow I had forgotten, or never known, that Citgo is full owned by Venezuela's national oil company, Petroleos de Venezuela SA, until someone reminded me recently in the context of wondering at how people could continue to do business with Hugo Chavez. So far, official U.S. sanctions on Venezuela go only so far as new arms sales, and existing contracts will be be respected until they expire. But one could argue that social sanctions are much preferable to legal ones; people who were unaware of laws against doing business with a particular country frequently get tripped up by them, whereas simply being condemned by others raises no problems of notice, due process and so forth.
It also doesn't create the potential First Amendment conflict that came to mind when I read about Republican presidential contender Rudy Giuliani's law firm doing lobbying for Citgo. If we put sanctions on Venezuela even beyond the level we did with South Africa (in 1986 Congress overrode Reagan's veto in order to ban the importation of South African goods and prohibit American business investments in South Africa), such that Americans were prohibited from doing business with Chavez's government or any state-owned Venezuelan company, could this include lobbying activity? As far as I know, the only groups with which we've actually banned all transactions whatsoever are terrorist groups, which generally don't have paid lobbyists to the U.S. government, though I'd want to double-check that with regard to the IRA and other organizations that haven't threatened the U.S. itself.
I would consider lobbying to be fairly core political speech, and one cannot easily be legally prohibited from getting paid for one's speech, so lobbying would seem to be a business one could continue to conduct on behalf of a sanctioned nation -- though if the country is at such a point of disrepute, the lobbying might be somewhat futile.
(Speaking of futility, an old question: should one be able to sell books advocating tax evasion? Despite my favoring First Amendment protection for books that tell one how to commit murder, I find Federal Mafia: How the Government Illegally Imposes and Unlawfully Collects Income Taxes less deserving of the same, perhaps because it's clearly advocating criminal activity, rather than simply being a guide on how to commit it.)