Due to the 1:8 coordinate multiplication behavior of vanilla Nether Portals, players can quickly travel to the end of the Minecraft world by repeatedly digging down into the nether and traveling back up by Nether Portal.
Apart from disk storage pollution the end of the world is also a severe lag issue for servers so it should be definitely avoided for public servers. For example set a world border and/or remove nether portals.

Can be considered solved by the Nether portal deactivation feature added to this mod.