Block current flow out of powered USB hub when supply is off
A Schottky diode would be the simplest solution but would cause a 0.3 V drop which you don't want.

Figure 1. Image source: SMPS 5V without load, fluctuation on load TOP258PN, uncredited.
Typically the SMPS is a transformer output with a diode and smoothing capacitor. The diode will prevent any high reverse current from your USB hub. There may be a couple of reasonably high value resistors on the voltage monitoring feedback circuit but these will leak little current and won't be damaged by the backfeed as, to them, it just looks like the PSU is running.
Recommendation: Don't worry about it. It shouldn't be a problem.