Hi Stefano
Am I correct in assuming you’re from Italy? I’m in Switzerland, and as most Swiss, I like to visit Italy once in a while! (Damn Corona!).
To manage NethServer and DNS, you can use the built in DNS Tool in Cockpit (Port 9090). To remove Hosts from AD, use Stephdl’s PHPLDAPadmin tool, from here:
https://wiki.nethserver.org/doku.php?id=phpldapadmin
Note: You will need to activate “Kamikaze” mode!
To clean out the AAA entry, try here:
nano /var/lib/nethserver/db/hosts
(I use nano as editor…)
Nano Quick tips:
Search: CTRL-w
Save & quit: CTRL-x y
Hostname / FQDN (Fully qualified domain name):
Even Windows has always differentiated between a simple hostname (With the stupid restriction of 15 CHR$ length) and a FQDN (Always also includes the hostname at left…).
If you only need a Hostname, just add in the standard domain (In your case .local) - that’s what Windows also does… Simple enough! 
You CAN use most Windows Tools to administrate AD in NethServer. Best bet is RSAT, that works very well and is quite compatible, at least for users and groups. Group Policys work well.
Example: RSAT AD / Forest tools won’t work, as that isn’t supported in NethServer…
My 2 cents
Andy