It is impossible for one Roku remote to control a Roku TV in another apartment unless the remote has a direct line of sight to the TV in question. IR will not pass through walls, and most Roku TVs come standard with an IR remote. And a WiFi Direct remote must specifically be paired with a device to be able to control it. That can't be done without being in front of the TV. It sounds more like someone has gotten into your WiFi and is using a Roku app on a phone or tablet to control your TV.
My first suggestion is to change the password on your WiFi, then reboot it, so anyone that has snuck into your network gets locked out. That means you have to reconnect all of your devices, but it would ensure no one has gotten access to your home network.
