First of all, contrary to internet lore, a small amount of light will NOT prevent plants from flowering nor revert already flowered plants to veg growth.
Cannabis plants flower fine under full starlight and moonlight, which can be fairly bright. Next, even if your PC fan hole did let a bit of light in, it would be a pretty limited amount, and that light would only be expected to hit a small part of the plant.
So unless you're literally shining a floodlight into your PC fan, I wouldn't worry about flowering.
The most important reason why you don't want light leakage isn't because of the small amount of light leaking IN, its because light leaking OUT of a "stealth" grow area is a dead giveaway.
How to fix it? Seal any pinhole leaks around the case with black tape, caulk, aluminum, foil, or any other light tight material.
The fan is a bit trickier. Basically you'll have to come up with a either a bent pipe or baffle system to let air through but prevent stray light from going through. Without seeing your case, its hard to say more than that, but this shouldn't be too complicated, and there is more than one way to skin a cat here.
A simple small box with an open bottom that fits over your fan may be an easy solution. You want the open side to be opposite the light, so air can get in, but light won't.