schneidz may be correct that a toram or other means to copy the OS to memory might allow you to remove the usb. The problem is that there are so many distro's and each way a live cd could be made makes it hard to guess an answer.
You can try it and see.
Some distro's call the boot option different than toram. Not all distro's have the ability to eject CD which is where this ability would come from.
A standard live usb that was a made from a normal install could not be ejected, only some live cd's using some live usb creator's could do it.