Builder used same brand for lights and fan in 3 different places
Changing the frequency IS the answer, regardless of the location of the fan. A remote with a new battery could easily work to a house next door.

Some remotes/receivers use DIP switches. They are tiny little switches in a row, usually four of them, and you change the combinations of on/off so that the receiver and the remote match. 

Others use an RF signal where you typically press and hold a button 'till an LED lights up, then press another button so that the two are paired.

You really need the instructions to let you know what method your fans use to change the frequency.