I think posting a fake wall of stone seconds before someone fires a ranged touch attack at you would probably not be that effected. As a DM i'd rule that he has a 50% miss chance but that's about it.
Depends on the spell. 50% miss chance is nothing to sneeze at either, but it would completely negate any spell that requires a target or targets.
"Some spells have a target or targets. You cast these spells on creatures or objects, as defined by the spell itself. You must be able to see or touch the target, and you must specifically choose that target. You do not have to select your target until you finish casting the spell." Magic Missile comes to mind. Also, assuming that you believably cast your "wall of stone" you may convince them not to cast the ranged touch spell, and instead switch to something to deal with your wall, such as a Rock to Mud.
As always, illusions are only as powerful as the imagination of your Dungeon Master - if he's an illiterate slug, he'll not be able to imagine someone believing in your illusions, and they will always suck.