Great plugin, but holograms can still appear next to blocks, and I recommend you to use armor stand entity with skull on it's head, because using head block can still remove other blocks that is entity standing on, such as carpet or slabs. That means when you will kill a mob which was on carpet, head block will override and remove carpet block.
http://recordit.co/xUARrkTJRb
(Don't think that I'm lagging, it's a GIF)