As I explained, killing existing leases from your DHCP server is likely to go completely unnoticed by those you want to cancle since as soon as you kill one lease, they just ask for another. But because the clients don't know about those leases being terminated, they do not ask for another lease but ask for their existing release to be renewed. Which is likely to be granted by dnsmasq.
So killing leases on the DHCP server might just don't do anything.
Just give it a try: SSH to your router, delete the lease file and restart dnsmsq. I bet you won't notice any disconnects and your lease fill will just get recreated and refilled as soon as your clients think their leases time out.
Why not simply turn off "Dynamic DHCP" in the first place?
It is described as "Dynamically allocate DHCP addresses for clients. If disabled, only clients having static leases will be served.".
- Turn off "Dynamic DHCP"
- Create static lease for all your known MAC addresses
Will result in: Only your known devices get IP adresses.
But of curse that only adds some levels of annoying to the clients you don't know, it does not provide any kind of security.