It's too late to freeze the current version of the browser. Browsers had a "kill-switch" baked in for some while, since last ~1.5 years. An older version (perhaps Mozilla Firefox may be easier to find and install) may still work.
Or manipulating with the dates on the machine.
That said, Adobe Flash does provide a way to "domain whitelist" Flash content after December 31 for intranet environments: https://www.adobe.com/products/flashplayer/enterprise-end-of-life.html. See "Enterprise enablement support" chapter. It wasn't planned originally but I think they included this capability sometime this July or so. Perhaps this should be your way to go.
Keep in mind when Adobe says "unsupported" it means they don't provide support, it doesn't mean it won't work for some time. It will - that's why this option exists. It will probably still stop working after a while, perhaps with the Operation System evolution (when the new OS version becomes incompatible with the legacy version of the flash player - and updates are not being created, except by that Harman partner they referred to).
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Dmitriy Alergant, Tier One Analytics