SteamOffline sets loginuser.vdf to offline mode and read only, so the Steam client automatically starts in offline mode.

The OfflinePatch is a very aggressive method, but the safest. It blocks your Steam and all it's executeables by the firewall for further support.

The download can be found on the Release page.

• install a second Steam client - Recommend using Steam Beta if you want to use second account
• login and install games you want to run offline
• close Steam
• Make sure "WantsOfflineMode" is in the Steam/config/loginusers.vdf If not go in offline mode once (Steam > Go offline), restart and close Steam.
• place the SteamOffline.exe and OfflinePatch.exe into your new Steam folder
• run SteamOffline.exe or OfflinePatch.exe if you want to Steam to be blocked by your firewall
• you can keep using steam.exe or SteamOffline.exe. SteamOffline.exe just makes sure that the loginuser.vdf is still in read only and has the correct parameters.

@ setlocal enableextensions 
@ cd /d "%~dp0"
if exist steam.exe (
for /R %%f in (*.exe) do (
  netsh advfirewall firewall delete rule name="Blocked: %%f" dir=out program="%%f" action=block
  netsh advfirewall firewall delete rule name="Blocked: %%f" dir=in program="%%f" action=block
)
attrib -r "config/loginusers.vdf"
)