From vim documentation:

If you make changes to a file and forgot that it was read-only, you can
still write it. Add the ! to the write command to force writing.

If you really want to forbid making changes in a file, do this:

vim -M file

Now every attempt to change the text will fail. The help files are like this,
for example. If you try to make a change you get this error message:

E21: Cannot make changes, 'modifiable' is off 

You could use the -M argument to setup Vim to work in a viewer mode. This is
only voluntary though, since these commands will remove the protection:

:set modifiable

:set write