Physical Ways to Stop a Seat from Reclining
Place a sturdy water bottle on your tray just under the latch.
After the plane has taken off and the “fasten seatbelt” sign dims, pull down your tray table. Place bottle directly underneath the tray table latch. You may need to use your book or something else to hold the bottle in place between the tray and the latch. Gently push bottle as far back as possible, into the seat in front of you. When the person in front of you tries to recline, the seat won't go back.
This may not work for every airplane seat, and it depends largely on the seat’s design.
Use your knees to block the seat from reclining.
In a pinch, you can always lift your knees up and place them on the back of the seat in front of you to prevent the passenger from reclining.[4] Just be sure to place them there gently, so as not to make the unsuspecting passenger uncomfortable, and be prepared for some cold looks or even confrontation if they peek back and realize what’s going on.
Point your over-seat air conditioning at them.
This one’s fairly petty, and we don’t recommend it unless the situation is dire. That said, you might reach over your head and turn the personal air conditioning nozzle to full blast, then point it forward at the reclining passenger to dissuade them from reclining.
Just as it’s their right to recline, it’s your right to use the personal A/C. That doesn’t make either action polite, though.