Pet feeding neighbour

I am the owner of an apartment in a small block. I own a cat, which I have permission from strata to keep. My upstairs neighbour (a tenant) has 4 cats, so is obviously very keen on them. Unfortunately, she has now started feeding my cat, and keeping her in her apartment for extended periods. I am not happy about this, but short of keeping my cat inside, I am unsure of what to do. I have attempted to address the issue with my neighbour, with no success.

My question is- is there some sort of law which stipulates that owners/occupiers are not to interfere with other owners'/occupiers' pets? Although it sounds a bit over-the-top, my neighbour is, in a sense, infringing on my property (i.e. my cat).

Any help very welcome!