Can I prevent a file from being modified?

Dear subversion,

I have recently started a new post and I am in the process of
investigating some possible new working practices. This particularly
relates to document control.

I have read in various places about the 'tagging' system (where by I can
use a property to effectively 'name' a file) used by subversion and
things such as svn:needs-lock that forces any user who tries to commit a
change to a file to be asked to acquire the lock that the file holds.

These almost fit my requirements, but not quite.

What I require is a method where I can make the file truly 'read only'
after a certain point.
 From what I have read in various places I thing I understand that...
subversion reflects the unix style permissions on it's subdirectories,
in the subversion directory tree.
If I want to make a specific location 'read only' I need to do this from
a new directory, and then files in this directory will acquire the
permissions of the parent directory.
subversion doesn't store the file permissions of a file directly it
stores it within the svn:properties

However if the above statement is correct this may not fill my requirements.

Let me expand on my problem (situation).

A group of users may be working on a file over time. At a certain point
they all agree that the file is up to its required needs and they
formally create a hard copy of this file, and store it as a controlled
document (signed and counter signed blah blah blah).
The finalised hard copy of the file must be an exact match of the copy
stored electronically. All the users will be aware that this file
shouldn't be change on the disk. But life being life when a user opens
the file to print it they may inadvertently add a space in a paragraph,
or an accidental carriage return, or worse

         how often have you been working on document1 when you get that
    phone call to ask about a specific part of the controlled document2,
    so you open the document to have a quick read and clarify the
    situation. Then you get distracted by something else and you return
    and start editing the document2, thinking you are editing document1.
    We all know it shouldn't happen but it does occasionally.

What I want to be sure of is that even if this is done by accident there
is no way that the file on disk in the subversion repository can be
inadvertently changed. The svn:need-lock property combined with another
personal property (something along the lines of CtrlDoc:DO_NOT_CHANGE
TRUE ), unless there is an svn:read-only property that I am yet to come
across!

I feel that the combination of these properties and proper use of the
Head, commit and branch strategies should enable myself and my users to
ensure that these types of problems don't occur.

Please help me clarify this so as I can propose the use of a subversion
to my colleagues, and give eloquent and correct answers to any of their
queries.

Thanks in advance.

David

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