people who live in countries that are under US sanctions i would presume? I'm not an expert on this stuff but I'd be surprised if buying or selling US securities was not heavily restricted for people in sanctioned countries like Iran.
Not really. Every government ultimately draws their legitimacy from the consent of the governed. If enough people riot and/or strike, you're done. Even Rome had a grain ration to keep the people content.
Your options for pressure usually come down to making the people of that country uncomfortable, or military action. Neither of which are very comfortable for those on the bottom.
You can also bribe the government to clean up their act, but that has a mixed record. I also wouldn't really call it putting pressure on the regime.