There is precendence for deportation and the stripping of US citizenship. There are laws against going outside this country and speaking against it. They haven't been used in a while for sure, but they could be. John Kerry should have been arrested and deported for meeting with parties in France that were negotiating for Peace with the US in the 70's. Jane Fonda should have been arrested. Jesse Jackson and Calypso Louie should have been arrested for going to the ME to meet with enemies.
Current US law and state department policy says that's not going to happen here. Specifically, a person must perform an action with intent to give up his citizenship. In most cases, that intent must be clearly stated by the person.
The only exceptions are conviction of treason, taking a policy-level office with another country (although not always), or the extremely rare instance where a person violates a law that can result in loss of citizenship and the government decides his actions in doing so were so outrageously inconsistent with citizenship that intent is obvious.
It's also interesting that even stating intent may not be good enough. Marc Rich (Clinton's favorite pardon) continued to use his US passport after officially renouncing citizenship and kept referring to himself as an American. The court held that meant he lacked true intent, was therefore a citizen, and thus subject to a certain lawsuit.