Have you deleted all the files from your wordpress installation (except for wp-config.php and the wp-contents folder) ? Do that, download wordpress again and do a fresh install. Delete your wp-contents/plugins and re-install all plugins. Make sure there are no dropped .php files in wp-contents and its sub-folders which might provide a backdoor. Change the MySQL Passwort, your Wordpress user passwords and for securities sake even your shell/ftp password. Change your passwords after you are sure there are no .php files in wp-content/ except your newly installed plugins (from the original source, re-download them). Do you have any other scripts on your account (besides Wordpress) which might have security hole? Also make sure that none of the plugins are the point of entry and install the latest versions.
You can also check your Apache logs if there is any suspicious activity and find the point of entry in them.
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