In K4Mac, the "updates" folder is located in User > Library > Application Support > Kindle folder. If you can't find it there, try: Application Support > Containers > com.amazon.Kindle
I replaced it with an empty txt file named "updates" (no extension), which seems to be thwarting update attempts. An update starts to download but cannot complete. Instead of auto-updating, K4Mac asks me if I want to download a new version (every time I launch K4Mac).
ETA: Per
jhowell's awesome tip, applying chmod -w to the "updates" folder also works.
Note: chmod -x still works on 1.30.0 as long as you remember to
right-click to download instead of open books, so getting auto-updated to 1.30.0 on Mac isn't necessarily a problem.