There is no function under Windows USB framework to suspend USB devices at will. This function was deemed to be unnecessary from Microsoft designer's point of view.
However, for testing purposes, there is a tool from USB-IF to perform SUSPEND (and wake-up) and many more isolated functions on USB ports, mostly to invoke various test patterns for electrical characterization. It is called xHSETT. Unfortunately, this function is implemented as a special USB test driver that replaces the standard stack and has no standard application functionality.
There is also a little nuisance with this driver that when something gets wrong and ports end up in some unexpected state, the test driver will fail to unload, and the system may stuck in this test mode without basic USB functionality. So you will have no control over the system with keyboard or mouse even after system reboot. To keep the system functional, it is advised either to use a test system with PS/2 port, or have an extra plug-in PCIe-USB card.