By default this plugin sends a chat message to the player when building is blocked. I would recommend instead sending the default Rust gametip:
I find having it consistent with the vanilla alerts is a bit more fluid, but maybe it's personal preference. A configuration option to change to the alert box style would be great.
If anyone else wants this, I was able to implement it for now by replacing the Player.Message method with a gametip in the alert style:
target.player.SendConsoleCommand("gametip.showtoast", 1, "You don't have permission to build here");