From the TextFlow documentation:
The wrapping width of the layout is determined by the region's current width. It can be specified by the application by setting the textflow's preferred width. If no wrapping is desired, the application can either set the preferred with to Double.MAX_VALUE or Region.USE_COMPUTED_SIZE.
So, to stop a TextFlow from wrapping:
textFlow.setPrefWidth(Region.USE_COMPUTED_SIZE);