And the reason to stop it is obvious-- to procure other taxes from those people. For example, many jurisdictions don't have an inheritance tax, and the US Gov't would likely want that cut upon the (ex?)-citizen's death.
>how could the US government stop this from happening.
Lots of simple solutions if we look at other countries that restrict emigration. USSR and North Korea had models that worked pretty well. High boarder security, labor camps, family punishment, and asset seizure.
These could all keep people working and generating tax revenue in the US when they want to leave.
On a practical level, I think the idea of coercing people to stay in the USA would be unpopular, especially when contrasted with the current wealth tax on exit.
