I have uninstalled the Apprentice version of houdini, and I want to get rid of the license server completely, how do I do that?

Easy,

First off, go to Control Panel > Administrative tools > Services.
Stop the following 2 services: Houdini License Client & Houdini License Server.
Now delete the server files from the Winnt/system32/ folder. They are named
hserver.exe and sesinetd.exe
The licenses themselves are in a folder called keys, but you can leave those.

Cheers

where are these key, cuz I want those gone too.

Winnt/keys

Hello,
we are a film school in Rome, we bought 8 licenses for iMac of Side FX Houdini v16.5.324 (Education), I have to format one of these iMacs, and then reinstall the previously mentioned version.
Which is the correct procedure so that I will not give error to the purpose of reinstalling the software on this iMac?
Should the licenses of this iMac be returned to the server before being uninstalled?
Thanks for the support.

Try deleting file licenses in C:/Windows/keys and reboot the computer.