Bitter orange extract, sold in small vials of liquid in pet shops as a training tool for cats - cats hate the bitter taste, and will avoid whatever it is applied to. Usually. They are cats, after all.

A more budget alternative is to use tabasco sauce, but please, in moderation. Both this things are very physically unpleasant for cats. Many cat handlers recommend only using them on cords - which have the potential to kill if the cat doesn't learn to avoid them - or forbidden objects which are unsuited for other methods, such as placing a pop can with a few pennies in it nearby, which scares the cat without causing pain.