Sic Semper Tyrannis doesn't change based on who the tyrant may be.
The "Entity no one could prove existed", I'm just gonna call the ghost for now.
Let's say for argument, that the ghost exists. A non-existent ghost doesn't sound like a problem, sounds like a good story.
Let's also say the ghost leaves no evidence, but can somehow still interact with the physical world, to bring about the ends it desires.
Let's say the ghost has communicated in a way that does not prove its existence, however it does indicate the intentions of the ghost, if the ghost were to exist. Such that someone could learn the ghost was an incubating tyrant.
Given the above premises, rejecting the ghost is the moral imperative. However, given the above premises, there would be no physocal action capable of delaying or stopping the ghost. So I guess the only action you could take, would be to publicly state your opposition to the ghost's desired outcome, when the ghost came up in conversations.
In this case, you do not need to even believe the ghost exists, to be opposed to the ghost's agenda. A fictitious character's agenda can still be held as abhorrent to an individual.