Is there a reliable way to prevent upgrading to 6.7?

Running 4.9 headless on an NAS and 6.7 won't run on it so I don't want to upgrade. I tried setting removing everything from the C:\Program Files\CrashPlan\upgrade upgrade folder and setting to read-only, but I still get upgraded immediately when starting 4.9. The upgrade was effectively prevented on the NAS with the same method. Now it won't even let me login if I lock the folder, it just sits at "Upgrading CrashPlanPro" forever.