How can I disable AirPlay on iPhone?

You don’t really “disable” it — you just don’t use it. AirPlay only activates when you choose a device to stream to, so if you don’t select one, it stays inactive.

Settings > General > Airplay and Coninuity > Automatic Airplay > Never [Tick]

The only option you have to is to go to Settings > General > AirPlay & Continuity > Automatically AirPlay and change the setting to Never/Ask/Automatic. If you don't want it to ever engage, choose Never.

To stop an active AirPlay session on your iPhone, open Control Center, tap the Screen Mirroring icon, and select "Stop Mirroring"