Here is a site that will help you determine if the plants you have are toxic to cats: http://amby.com/cat_site/plants.html
It isn't what is used on them it is the plant itself. If they are on this list, simply get rid of them.
If they are OK, I've used 2 methods depending on the size and shape of the plant: Make a grid over the top of the pot with masking tape - tape over the edges and between the plant so that they can't get into the dirt. For large pots, I use chicken wire as the grid. Your other option is to confine the plants to a room where the cats don't go and close the door.
I've hung plants from the ceiling and the cats have still managed to climb on top of something tall and leap over to them. Many cats like to dig in dirt and if they want to get to them, there isn't much you can do to keep them away. They are just too compelling to them.