(1) You go after the companies collecting the money (Visa etc) and delivering the app (Google, Apple). These very much have physical offices and principals in the US.
(2) You arrest people who dare to fly through/over/near the US. See: legal UK gambling companies.
(3) You attempt to extradite anyone not caught by (1) or (2), if necessary pushing for one-sided extradition and IP laws to do so.
1) Yes, also I guess they could order ISPs to block the website, but I don't know if there is any precedent for this.
2) No, patent infringement is not a crime
3) No, patent infringement is not a fellony (or crime).
If there was a Default Judgement against them then violating it may be a contempt of court, which may be a crime - anyone know if that's the case?
Edit: Wikipedia says "Contempt of court in a civil suit is generally not considered to be a criminal offense".
http://finance.yahoo.com/news/jailed-for--280--the-return-of...
http://farquharlaw.wordpress.com/2012/04/26/debtors-prison-e...
In the Yahoo Article, the woman arrested failed to show up to multiple court hearings, despite receive notice at the proper address. The fact that the court hearings related to a debt was incidental. Blame shoddy journalism.
The second one, is a blog post, which actually talks about something entirely different from a debtor's prison. It refers to a "debtor's exam", which is a fancy way of saying that your finances are examined to see if you can afford to pay the debt. No prison time involved. Intrusive maybe, but warranted if you haven't paid the debt at issue.
http://www.nytimes.com/2009/04/06/opinion/06mon4.html
http://www.nytimes.com/2012/07/14/opinion/return-of-debtors-...
http://online.wsj.com/article/SB1000142405274870439650457620...
Debtor's prison is alive and well in America.
Just preventing sales from customers in the US app store would be a lot of damage. Google and Apple already have different products available according to country of origin. Billing addresses can also be easily checked.