Help, Network Table repopulates with devices that don't exist anymore

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I am not experiencing issues with Pi-Hole itself, but just with a feature of Network Table

Expected Behaviour:

When deleting devices from the network table, the devices SHOULD NOT reappear in the table if the physical devices don't exit on the LAN anymore.

Actual Behaviour:

I purchased a few smart-plug devices that I tried to install on my LAN and they obviously took a DHCP IP from the router. Unfortunately they did not work with HomeKit and I decided to return them to Amazon.
Their MAC, IP and Name records were added to the Network Table. When I delete these lines, they keep reappearing. I even flushed the Network Table, but these device keep showing up after a few seconds.
I wish I would understand from where is this information coming back to the table if the devices are at Amazon???

I have been told by moderator jfb that this is coming from ip neighbour table.
Those device were in that table, so I flushed the neighbour table as well by typing:

sudo ip neigh flush dev eth0

Them I flushed the Network Table again, restarted Pi-Hole
Unfortunately these devices come back to the Network Table in Pi-Hole.
Can someone explain to me from where is this information coming back? And can I stop that?