Using EMI to cheat items while in creative mode will work regardless of permissions.

Pretty sure this is working as intended. Vanilla gives the client authoritative control over the contents of the inventory in creative mode, so a modded client can spawn in any item it wants.

This is correct, modded client or not, vanilla clients can use the "saved hotbars" to bring any item, including command blocks with preset commands, nukes, or whatever, into a server if they have creative but no op. So, there's no reason to limit EMI cheating.

This is unfortunately causing issues with Minestuck, as it puts players in a creative-like mode, but with limited items.

In that case they should likely do one of two things:

• patch the server on their end to reject attempts to spawn in foreign/blocked items
• use survival mode and implement a custom creative-like interface on the client (however, the server still needs to validate what the client is sending to it, so this ends up being similar to the first option - it may be less buggy though)