The wiki is lying. Not all piety goes towards reducing gift timeout, and gift timeout is reduced even when piety taper stops the piety gain anyway. Sif's gift timeout is 40+random2avg(19,2), so you miss an average of 16.333... piety as a result of a book gift, provided you stay at or above 160 piety for the duration.
A "turn gifts off" option is something that only spoiled players could use effectively, since Sif is pretty much the only case where it would ever make sense to use it (gifts do not deprive you of very much piety at all). Everyone else would be hurt by it.
Anyway I wouldn't mind Sif gifts stopping once all spells are
available. All spells being
given is different - a book gift received in the Abyss, Pan, a portal vault, etc. and left there is no longer accessible, so after you've been gifted all the spells in the game, you not actually have all of them available to learn. So if this is implemented, it should check which spells the player can currently access without buying them from shops, not which ones have been given. But it's probably not worth adding the extra complexity in the first place.