How to make F-Droid hide data about incompatible packages?
Usually, this should be what the corresponding toggle on the Option page is for: If you untick "include incompatible apps", they should be hidden (and if you tick it, they are shown but grayed out – just as you describe).

As this doesn't work for you, you might be plagued by a long-standing bug. Unfortunately, there's nothing we at Android.SE can do about it but suggesting you to chime in there and rise awareness, so it might finally get fixed. As the issue is still open, it seems so is the bug – though updating to a more recent version is worth trying (v1.2.2 is not quite up-to-date; ATM v1.6 should be the current stable) if your Android version permits.