This is such an old Idea, and I'll explain the reasoning behind why it isn't implemented: You buy this item for the free AOE damage it deals. Suppose you are in lane, and you are using the free damage to drain away your opponent. Logically, they ought to turtle under their tower, right? They are able to counter play you by taking advantage of this items particular weakness, namely, drawing aggro. If you can toggle it off, it becomes OP, unless it deals almost no damage at all, for the simple reason that it removes all the possibility for counter play.
I for one would much rather have an impactful damage source than a totally negligible one that has a toggle. Right now, they need to buff sunfire's damage scaling to something like 2 per level instead of 1, but other than that, it shouldn't have a toggle. You bought it for the damage, so you pay the price for it too, just like an ADC pays the price for building damage by being the primary target in a fight, you pay the price for damage by drawing aggro.