Ignorant trash picking my flowers/plants when I leave, what to do?

I live in a neighborhood where apparently there are trashy/ignorant neighbor's who don't respect others property.

I spent about 2 months busting my ass, preparing my mom's new yard with different plants/flowers. I planted a large flush of wildflowers (dozen + variety) at the top of a 4' slope (which you would need to climb into the yard to get to).

I was showing her these nice peach wildflowers that were just starting to bud today and we were talking about how tomorrow I would come by with a camera so I could try and identify them.

Lo and behold I leave for 2 hours and she calls to say someone came by and just took them all out. I guess to make their own cuttings or an arrangement?

Either way...how fucking rude is that? Human scum. Is there anything I can do besides a "no trespassing" sign? Anyone else have this problem and any solutions other than me 30 days in county for assault?