The house I recently bought (over 100 years old) has had almost all of its wiring replaced with a new elctrical board. However, in two cases the new wiring has been connected to existing knob-and-tube wiring in a junction box, with the ground wire not connected. So the knob-and-tube wiring is still in place, fed by new wires coming from the panel. Can I just disconnect the knob-and-tube wiring, leave it where it is, and bypass it with new wiring? If I can leave the disconnnected knob-and-tube wiring, do I have to label it somehow?
Yes, you can leave the knob-and-tube wiring in place. Labeling is neither required nor common, but if things are confusing enough that you think it's warranted, it can't hurt. It's also a good idea to rip out whatever wiring is accessible (e.g., in an unfinished basement).
If you have an electrician do the disconnection work, you should ask the electrician to sign a form certifying that all knob-and-tube wiring has been removed. Such a form is sometimes required before blowing in insulation, or when selling the house.
The typical process of disconnecting an unused wire while leaving it in the wall is to join the hot(s) and neutral at each end so that it's obvious that it's unused and any attempt to reconnect it would trip a breaker. And I hope that no crazy electrician would ever want to reconnect an old knob and tube line. I would take the extra step to completely remove it from anywhere that it's easily accessible.