Maybe some people who are used to dealing with toddlers have some good ideas:
A friend of mine has a problem. (No, really.)
My friend's neighbor, who is widely acknowledged by the neighborhood to have a drinking problem, has been coming into my friend's garden and either:
1) cutting my friend's flowers and taking them home or
2) pulling up whole flowering plants by the root balls and replanting them in her own garden
She is not, nor has she been at any time, invited to do this. The neighbor has been asked, at least three times, not to take the flowers. Most recently, she was told quite loudly and forcefully to put the plants back. She continues to wait for opportunities when my friend is not home and then raid the garden.
Their yards are adjacent and there is no fence. In this particular neighborhood, fences are quite uncommon.
What would you do? Call the police? Yell and scream? Plant some poison ivy in your flower beds?
I think I would put up a fence. It's an expensive solution, but it makes the trespassing issue clearer to everyone (not just those directly involved). And the fence will be a nice backdrop for whatever flowers are left.