imagine an advertisement so effective that anyone who saw it would immediately buy the product at whatever price asked as long as they could afford it.
Imagining this we can see that there is obviously some limit beyond which advertising must be curbed, and admitting this the question just becomes at what point must advertising actually be curbed?
Of course one could argue that such an advertisement must be for a product so wonderful and useful that everyone would want it - let us say immortality with youth and good looks - but if that were the case such a product should be recommended enough by word of mouth and the evidence of all the old people becoming young and good looking when taking it.