All of the aircraft that I am familiar with, have the hydraulic actuators, as previously mentioned, for seat recline.
These actuators have a locking device with them, simply take out the locking pin which is in a stowed position on a serviceable seat, and fit it into another hole which will lock the actuator and thus the seat back.
I have also done this sometimes when travelling with an aircraft, with the Pursers knowledge, to stop these people (as previously mentioned) that recline the seats on purpose before takeoff
IF you are on a long flight, this locking pin could be very quickly removed again for the flight enabling recline, and then put back again for landing.
