@savath 

There is no related settings on the router, I assume it is a function of smartphones.

To protect your network, you could consider using access control whitelist and add your own devices MAC address into the whitelist only, other devices not listed will not be able to connect. Here is the guide: How to configure Access Control on the Wi-Fi Routers

Note: please make sure your devices are not using random mac address, or it will change randomly and then unable to connect when MAC address changes.