I have been told to bury wire (vertically) a good 12" below the external soil line to prevent digging animals from getting in, and to attach boards on deep stakes/rebar or (as you mentioned) cement blocks at the bottom to deter digging animals. We actually did two different things (2 coops on different properties). I'll tell you about them... just in case it's helpful
For the city coop/run I have boards along the ground and wire walls wrapped under and stapled to the boards with electrical staples (heavy duty). I'm digging a narrow 2" wide x 8"+ deep moat around the run and just pouring cruddy cheap concrete to provide a rust-proof barrier to diggers.
For the county coop (more predators in the neighborhood) we dug out the yard and put down chainlink (on the ground) under the entire coop and run, and attached it to the walls of the coop & run... so that nothing can dig up from underneith. Then we buried it with all the dirt we dug up plus some rock/gravel/pebbles that we had on hand/on the property already (so that it's raised slightly from the surrounding ground- keeping water from pooling, hopefully). We will cover this with some sort of litter for the birds to run/scratch around in and rake out periodically for the compost... probably sand in a portion of it and straw/pine shavings in the more covered portion.
The city coop has motion detector lights around the outside, to both wake up the birds (who will wake us up- being 20ft from the bedroom window) and startle/call attention to predators at night (works on loose dogs and probably raccoons- the local cats haven't set them off so I assume a possum wouldn't). This doubles as a house security measure, as it's an urban environment and the backyard is rather dark. Thinking about doing this on the other coop as well. The only issue is making sure the birds themselves don't set the detectors off... so that they will go back to bed... lol. And... we might have a rooster at the less urban coop... in which case we might *not* do the lights... since we don't want to wake up the entire neighborhood if a leaf blows by (it's not within city limits, but just being neighborly).
Agree about the locks- we just have the same masterlock on everything (bought a set of identical keyed locks) and an extra key on the keychain, plus one extra key hanging on the coatrack nearest the back door. No big deal there.