I can think of a number of ways.
From arresting you at the border next time you enter the USA,
to just not renewing your passport and thus forcing your return to the USA.
For most residency visas in other nations are tied to you having a valid passport.
Or making financial hell for any bank that has you as a customer and does not report your information to the IRS, or maybe even even with hold taxes for the IRS, which is exactly the intent of FATCA.
Thus the assumption you can still find a bank / finance institution outside of the USA willing to work with you in itself maybe flawed, for increasingly expat US citizens are denied investment services and accounts by banks outside of the US. For you are too big a risk, which could theoretically cause them 30% of their US assets being witheld.
So, if you were willing never to set foot in the US again, risk living in a third world mud hole as an illegal immigrant, jobless, just doing cash transactions and hiding your fortune in a matress....
-Cynic