I see. Sorry for my confusion.

One way to do it is this:
1) Remove the following from the door (these make it locked, needing a key):
<inherit name="container_lockable" />
<autounlock />
<autoopen />
<locked />
The first one is an inherited type. The other three are attributes.
2) Add a script to handle the open case. Here's an example:
<openscript type="script">
msg("No matter how hard you pull, the door won't open.")
</openscript>
You will probably want to expand it to allow the door to be opened once the condition keeping it closed is gone.