i don't know if this works for cats... but in the more forested areas with bear problems guys will pee outside on trees to mark there territory to keep bears away
i can think of two things...
one:
threaten your neighbor with lawsuits with damage to flowers/screen door and or anything thing else you can think of
two:
get a cardboard box, put air holes in it, grab the cat, and it it on her porch, she'll get the idea