I have noticed that alot of people do not secure their racks a trunk bags while commuting. If you have the option to take off your trunk bag and take it with you, thats the best solution. If your like me and dont have anywhere to put it at work, here is what I have come up with.
I first soldered the 4 screws that hold the rack to the bike. I have tried to get the solder out without heat and it is virtually impossible.
Next, I used an old chain and inner tube trick to secure the bag through 2 small holes I put in the bag. I know what your saying... "You wrecked your bag, it will just rip and they will take the bag anyway, etc". Not true. In the bottom of my bag is a hard cardboard floor in between the layers of the bag. I also bolted the bag through that cardboard with a fender washer in the bag to the rack. It is not going anywhere.
I also use a small TSA lock to lock the zippers closed.
Now this setup is still vulnerable to knives, bolt cutters, etc. This is just a technique that was free for me and provided a HUGE amount of security over the bag just Velcro'd to the rack.
LOL at my camera phone pics. The phone is 6 years old and is still perfect for me. Not the quality pics of an iphone though.