It's not a bad idea to put an inscription on it, but my personal favorite is a talking face, then you can roleplay telling them to piss-off. If they attack the door, have it scream, and do psychic damage to them.

Honestly, it's just a terrible idea to put anything in front of your players that you don't want them to interact with. You don't have to. You can just let it be a wall, a perfectly seamless wall. Then later, when you want them to return, have them discover a map, or have a mysterious figure send them a note. Or better yet, plant a note on one of their enemies, so they feel like they won this new, secret information.

It's not a bad idea to put an inscription on it, but my personal favorite is a talking face, then you can roleplay telling them to piss-off. If they attack the door, have it scream, and do psychic damage to them.

Honestly, it's just a terrible idea to put anything in front of your players that you don't want them to interact with. You don't have to. You can just let it be a wall, a perfectly seamless wall. (Note: Do not describe the wall). Then later, when you want them to return, have them discover a map, or have a mysterious figure send them a note. Or better yet, plant a note on one of their enemies, so they feel like they won this new, secret information.