Controlling Devices on My Xfinity home wifi network

How do I:

a) delete devices that I don't recognise without having to disconnect them first (since, obviously, I don't know what they are)?

b) assign a permanent IP address to each device that I do want connected to my home wifi network.

In my last network I could always simply delete devices from my network, assign permanent IP addresses to each device (based on their MAC address) and name each device something easily recognisable to me rather than the usually weird and/or unrecognisable name of the device when it connects.

I want to be able to regularly review and control my network to prevent any unwanted or extra devices from connecting or remaining connected to my network. It's a security thing! 

Cheers