In hammer editor how do you make a room you cant noclip into?

There are two ways.

The first would be to create a normal room with walls of 16 units thickness, then make a brush textures inside the walls with the trigger texture and tie it to a trigger_hurt (ctrl+t). Double click the trigger to change its properties and set damage to 1000000 or so. this will make it so that anyone who tries to move through the wall is killed instantly (Unless they use godmode.

You could also tie the trigger to a trigger_teleport, make an info_teleport_destination outside named teledest_1 and edit the trigger_teleport so that the remote destination is teledest_1. This will move anyone who tries to noclip through your walls back outside.

If you want to kill players who try to move into the room but you don't want them to be able to use godmode then change the trigger in the walls to a trigger_multiple and create an point_clientcommand entity and name it clientcommand. Then go to the trigger and in its settings go to the outputs tab and make a new output. The output should be:

My output named: On trigger

Target entities named: clientcommand

via this input: command

With parameter override of: kill

With a delay of: 0