Is +5VSB standby power a feature of the "motherboard" or "Power Supply?"
If your mobo doesn't have jumpers to disconnect 5vsb from USB and other ports, then there may be a setting in the BIOS setup under Power Up settings, etc. If you have your mobo set to power up from a signal from Mouse, LAN, kbd., etc. then it stands to reason that there has to be 5vsb supplied to those ports. Some mobos have jumpers to supply 5vsb, others may have separate BIOS settings for supplying the power connection.
All ATX PSUs supply some level of 5Vsb to the system, but the mobo controls whether that line is connected to ports while the system is nominally shut down. The 5vsb also keeps the CMOS battery from being drained all the time, so it may not be a good idea to use a PSU's or power strip's on/off switch to totally disable the 5Vsb.