I recently ordered the Deco X55 wifi 6 home mesh wifi system. Install was easy and I've set up all 3 nodes in my home and connected a few devices manually through the app. 
 

My question is regarding other items connecting automatically. I'm getting notifications in my phone saying "xxx device has joined the network." These are a range of devices from smart home switches and plugs to amazon fire stick and TVs. Occasionally it will be devices that I'm not familiar with too. 
 

I'm not manually connecting these devices to my new network nor have I approved them.

How are they automatically connecting to the new network without me putting in the network password for each individual device? 

How do I stop new devices from automatically connecting?