Adjust forehead depth on the hair if it has the option.. it's usually depth sometimes forehead width. It's the base of the hair poking through the skin or something like that.
That's how I learned to fix it for renders, eliminating any need for post processing (although on very few occasions it might not stop the black lines/circles on the head skin for a certain hair/model combination). The issue is the Skull Cap for the HAIR isn't covering the model's forehead big enough to prevent "bleed through" of the hair asset through the model's forehead area (front and/or sides).
If you haven't tried it and want to see it "working" PRIOR TO rendering the scene, set your Aux Viewport to display iRay, then make sure the HAIR item is selected in the Scene Tab, then go to the Parameters Tab (for the HAIR) and left mouse-click and hold while on the "Forehead Width" slider. Slide the slider, and while you are doing it you will SEE the SKULL CAP on the model in the Aux Viewport iRay display get bigger/smaller. In most cases you will need to slide the slider to the right (increasing the positive number) to get the skull cap to enlarge and cover the entire forehead area. And as mentioned, you might also need to do the "Forehead Width" one a bit too (but I rarely need to do that one). Depending on the hair and model, it may only be a very small adjustment, or maybe a somewhat larger "slider to the right". Go just far enough to get the forehead covered by the Skull Cap and you won't see any change in the model's head hair dimensions when you render the image. You can still use other hair sliders or presets for the hair if they exist to manually shape the hair after adjusting the Skull Cap if you want to before rendering the image.