Closing off a doorway between apartments but keeping the door?
Standard method is to put a door with a deadbolt on both sides of the doorframe, so both parties have to open their door for the doorway to be open. If not anticipating much use, you might want to pack the space between doors with insulation to reduce sound transmission. And if you don't want them playing with it, no need to give the tenants a key to that particular deadbolt. No need for a knob, either.

Otherwise 2-3 studs, drywall, insulation (again, to block sound as much as possible) and more drywall. Or 4-6 studs (2-3 on each side) that are not connected in the middle for better sound blocking. Double-layer drywall on at least one face might also be worth it for acoustics.