Usually I just unmount the usb stick or mmc before unplugging it but I'd prefer to turn the power off before doing so. I think Windows does this, so the hardware should allow for turning off the power to a specific port but I don't know how to do this on a recent linux kernel (I am not sure if suspending is really the same but afaik it is deprecated/removed from recent kernels anyway). If you know how to do this, please explain. :)
There is really no reason why you would want to do this: pulling the stick/card out kills the power already, so turning off the USB port would be redundant. Also, how would the computer know when to turn the power back on, to prepare for another device being attached?
There is a related question at http://stackoverflow.com/questions/1163824/linux-usb-turning-the-power-on-and-off. To summarize, you generally can't turn the power off on ports off a motherboard, though some hubs might support this. USB has "suspend", but this is just your computer telling the device to turn itself off, power is still given through the USB port.