Once a corner (or whatever place) has the slightest trace of scent from a previous use as a peeing place, it is a magnet to your cat. You can scrub and scrub, but it's very difficult to eradicate. You won't be able to smell it anymore, but your cat's nose is vastly more sensitive than yours will ever be. Luckily, there is an easy answer.
Feed them where you don't want them to piss, JTR. Three or four meals delivered in the corner in question makes them see that as an "eating place" and cats never pee in "eating places". I find it best to provide the meals a few days apart. Let's say I feed them there on Monday and Tuesday (leaving the plate in-between times so as to provide an on-going reminder), then go back to the normal place on Wednesday, then once or twice through the rest of the week switch back to the corner in question. This fixes it in their minds as an "eating place". If you want to be really sure, deliver a few little between-meal snacks there as well (a saucer of milk or something).
