Under Windows you can disable the Internet access of ARES Commander 2016 (and following versions) via the registry setting:

e.g. for ARES Commander 2017 x86 and x64
HKEY_LOCAL_MACHINE\SOFTWARE\Graebert GmbH\ARES Commander 2017\internetaccess = 0
​
internetaccess is a DWORD value (32-bit)

Please note, that this setting will affect the licensing behavior of ARES Commander 2017 and newer releases.