Can I disconnect a neutral whose hot is disconnected?
Be very careful.
If you disconnect the neutral from a device while the hot is still connected then the disconnected neutral conductor will become live.
Hots and the corresponding neutrals are normally supposed to run together and neutrals for different circuits are supposed to be kept separate. However just because something is supposed to happen doesn't mean it always does happen.
What I would personally do is turn off all power (not just the circuit you think is involved) and disconnect and cap-off the rogue Neutral. Then turn the power back on and go round the house to see if anything stopped working.