How can I make some pages non-writable?

There are two ways:

Use the ACL feature (recommended)
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