They must be mounted to poles just like traffic lights. Then, people can't mess with them. Or just do my favorite method: narrow the shit out of roads until it is physically impossible to speed. Make the sidewalks and protected bike lanes massive, and the curbs lined with beautiful, reinforced steel poles painted to match the city they are adorning.
Throw around some steel planters that burst with flowers and trees, and also happen to weigh over a metric ton. Add BRT lanes with buses that carry cameras that automatically ticket cars in the bus and bike lanes. These problems are quite solvable and have been solved in NA and EU cities already.
The best thing is that these changes open a city up to being walkable and really pretty, especially with lots of greenery and landscaping being put in on previously busy roads. We have giant granite boulders and native plants on several of our major roads, now. It's done wonders to control speeding. :)