The fix is to create a Windows Firewall rule that blocks Rocksmith from connecting to the internet. If you already know how to do this you don’t have to keep reading!
Search Windows Firewall and select the first option:
Click Advanced Settings to open the firewall rules.
Click “Outbound Rules” on the left side, then “New Rule…” on the right side.
You should now have a new window. Click “Program” then next.
Click “Browse” under “This program path” and select the rocksmith.exe file.
Usually located under Program Files(x86)/Steam/steamapps/common/Rocksmith2014/rocksmith.exe
Press next and select “Block the Connection”. The last step after that is to just give it a name and press “OK”.
Congrats you can now start rocksmith!