How can I make some pages non-writable?

There are two ways:

  1. Use the DokuWiki ACL feature (recommended).
  2. Just remove the write permissions for the webserver on that particular file. eg1): 
    chmod 644 file.txt

    Be sure you understand permissions, as permissions impact security.

1)
The following example applies only for Linux/Unix like OS and assumes that the file is not owned by the webserver.