A partially deflated capped water or soda bottle put between the seat and tray table let's the person recline recline without letting them discomfort you by over reclining
The trick is setting this up BEFORE the other passenger reclines. You're not supposed to recline your seat or have tray tables out during takeoff. All too often, the former rule is substantially easier to break than the latter without getting noticed by the flight attendants. So in practice, immediately after the flight attendants sit down while the plane is still taking off, said obnoxious passenger reclines their seat all the way back, even though they are technically not supposed to yet.