How to svn ignore a file already added to the repository?

how can I ignore a file already added to a svn repository?

I like to exclude the file config.php:

$ svn st
M      .
M       config.php
M       adminfunctions.php

So, I use:

$ svn propedit svn:ignore .

And added config.php to the list.

Then, if I do another svn st, I get the same of the first time:

$ svn st
    M      .
    M       config.php
    M       adminfunctions.php

Wouldn´t the config.php be excluded just now?

Thanks in advance!