It's hard to remember to do, but shutting the door works well
My brother's cat kept sneaking into my room, and she would pee on things, so I wouldn't let her in there...I kept yelling "NO." at her and eventually she started listening...she would start begging "Mreeeeoww????" at me whenever I yelled, and then she'd leave. But I think that made her like me less =/
I like the penny idea better, wish I would've known about that. I'd like to try it and see if it works on her.
The laser pointer idea works great too, if you have one.
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I'll brave the storm to come, for it surely looks like rain...