(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.