It sounds like you need more sticks, or a covering of angular, pointy rocks. Half-crumpled chicken wire can also be effective. Some organic dog and cat repellents are also available, but they are of variable effectiveness--particularly now that she is in the habit!

Orange peel? Gravel?

If its the kind of cat who doesn't go far, or is nervous, then there's nowhere for her to go other than the bit of soil in your window box. Might be better to provide alternative toilet solutions for her, such as a litter tray, or a cat litter tray with holes drilled into it and filled with compost or similar and left outside on the patio for her to use. Poor creature...

A mulch of dried holly leaves. The 'ouch' should do the trick. Likewise plastic picnic forks planted with the prongs upwards.
Haven't tried these myself, just tips I remembered as being cheap, harmless(ish) & easy.

Cheapo pepper from the supermarket might work, especially in a window box. Any old cat might decide to use your window box as a loo, not just your own. I have Spooky, Blobby, Grumpy ginger and Calico Cat. I don't own a cat or a window box. If I did, one of them would probably make use of it.