There's GriefPrevention. I don't know id it's added to Aternos. If it isn't, just make a plugin request.
(https://www.spigotmc.org/resources/griefprevention.1884/)
Besides World Guard should be updated soon.
And you can even make a spawnprotection with command blocks only (https://www.youtube.com/watch?v=X2f1j3reXqs)
Thanks corbsmaster! Command blocks work perfect for me!
