You should try to use and internally threaded drop-in anchor or internally threaded adhesive anchor insert and then use a tamper resistant screw. Commonly "snake-eyes" or a regular style screw head (torx, hex, etc.) are availible with a pin the the middle so a regular driver can't be used to remove the screw.
There is no product the is vandal "proof". What is available provides differing degrees of resistance to tampering. You have to decide what level of resistance that you need to have and then make a decision based on that. The differing proprietary drives have differing lavels of resistance depending on how difficult it is to buy or make a driver. The Tamper resistant Torx Plus is pretty good because it has an odd number of lobes so most regular drivers can't be modified to fit. If you combine something like that with an adhesive you have a system that is pretty resistant to tampering, but given enough time it can still be taken apart.
By far the most resistant connection involves the use of a welder. There is nothing like fused steel to deter most vandals. But once again, given enough time even that can be defeated. I've seen what a guy in prison with 5 years of work can do to a welded joint.....
Most of the rotary broaches we make for security solutions are a hexalobular form or custom form the customer specs for his fasteners. We make the broach, internal or external, and they use it with the holder to produces 100's or 1000's of fasteners which use that form. The picture from the product search page of our website (below) shows a variety of broaches and products made using this principle.
The video on our website explains how to do it on a CNC or screw machine. It seems the medical community is starting to specialize in these fastener options to control their niche with bone screws and other orthopaedic products.
Is having the contractor damage the threads too low tech of a solution?
The company I work for has a standard note on many anchor bolt details, that calls for upsetting the threads after the base plate is installed.
I don't really know what upsetting the threads means. However I have never gotten a call from the field from a contractor asking me how to upset the threads. Maybe they just hit them with a big hammer.
These are all for stainless steel signs for a park in new york. I have seen too many of these things vandalised and wanted to see what the options were.
If you're just looking at holding a sign in place, a relatively inexpensive way to prevent someone from unthreading a fastener is to use a longer bolt than necessary. Saw a slot in the end of the bolt down to the point where the back of the nut should stop. After threading on the bolt (probably with some effort if the bolt isn't well deburred) stick a chisel in the slot and pound on it to spread the end of the bolt. It will be almost impossible to remove the nut.
they do a two part nut that has an inverted conical center with a matching outer ring. The outer ring is laser welded to the center cone, when you torque the nut to the specified load - the laser welds self destruct and you have reasonably secure joint. They proclaim to be selling this fastener to Power pylon companies for use against vandalism.
