My neighbor has two cats who were indoor cats, but she now lets them out during the afternoons. They enter and exit through an upstairs window which also has easy access to my roof terrace.

Normally I would have no problem with them, but they are using all my plants as litter boxes. The smell is disgusting, and I am constantly cleaning up dirt from the floor.

Is there a something that can deter them from using my pots as litter boxes?