I had a problem with this at a property I lived at before I got into the industry.
Our solution was to convince the owner to remove the actual rocks in the landscaping and move toward a less solid-object form of landscaping (pea gravel and bark mulch with groundcover flowers).
However, residents will find ANYTHING and everything to block doors with if they can. I might suggest sending out reminders regularly, and if you actually catch someone doing it giving them a verbal warning, written warning, and then potentially charging them (if your state allows) if they fail to comply (continued compliance violations resulting in more permanent legal actions, if need be).