1. Use the fastest acting fuse you can find- one for each device.
2. If you are using linear regulators as the final output stage, just find a linear regulator that has current limiting set at 500mA and use one for each USB device. The REG103 for example.
Hint: The current limiting value in a linear regulator is usually what makes it "500mA", "1A", or "2A", etc. After finding one, check the datasheet of a linear regulator labelled as such and look at the current limiting threshold to confirm. THe REG10x series for example, is basically the same silicon die with different current limiting values. The lower limit devices tend to come in smaller IC packages since they are expected to dissipate less heat, but you can find 500mA and 1A devices that both share the same package with different limits that are otherwise identical.
3. Current limiting can be built into switching regulator which will lower the voltage if the current exceeds the limit in order to keep the current at the limit (things like motors will keep running with reduced power, but most electronic circuits will simply stop working because the voltage is too low). This would require current limiting in the feedback loop of the buck converter...and would require a separate buck converter for every USB device.