Hey Fizzly !
On my end, the Cancel Build seems to work correctly. You have to put it in a common event and enter that common event ID into the "Common Event ID Before" parameter of the "Enable Build Mode" command. This should prevent the tile from being placed.
Note that the "Cancel Build" command won't cancel the whole build mode state, like pressing ESC does. If you wish to do that in an event, you can either call the "Clear" plugin command or use a script call with this script inside : Drag.SandboxMap.clear();
Let me know if that help !