This is not a duplicate of "What is this block I can't break??" questions. I know all about bedrock, including the Dark Oak sapling trick and the half-slab/wither skeleton skull trick. However, these tricks are not working on the Spigot/Bukkit server that I play on. I found something called Void Guard that sounded like it might be the culprit, but the guy that runs the server said there weren't any settings for that. What settings do we need to change to allow these tricks to work?
It looks like they patched this in Spigot builds as of January 20th. https://github.com/SpigotMC/Spigot/commit/d8ad596ece8c5414a77271deff0314691886339a
If your only reason for using Spigot is to have a dynmap, I'd suggest switching to a Bukkit server instead. Spigot is actually just a branch off of Bukkit anyway, and it's really only useful if you have a lot of people on your server or your server is on a really slow computer. Using a regular ol' Bukkit server will allow you to load server-side plugins just as well (dynmap, Essentials, WorldEdit, etc.), and it won't have as much code interfering with the vanilla-ness.
It says you need to set the value
Please refer to the site itself for the rest of the instructions, as it seems it is capable of keeping people out of the void areas after a hole is created.