Before you do this, you should make absolutely sure that this is what you want to do. If you just want to disable the site, then you should just use the option in the Reseller Control Centre to do so.
First, make sure you have SSH access for the site.
Then, ssh in as the site and make sure you're logged in as the right user.
Then you can type:
cat > .ftpaccess
<Limit WRITE>
DenyAll
</Limit>
...and then hit Ctrl-D to finish writing the file; the effect should be immediate. To re-enable FTP write access, you just need to SSH in and do:
rm .ftpaccess
