The actual FTP command is DELETE <filename>, or can be shortened to DELE <filename>. Or, if you are actually using the PHP functions (i.e. ftp_connect() and so forth) there is a handy function, rightly named, ftp_delete().
http://www.php.net/ftp-delete
Hope that helps.
Chris King
NOTE: This is all also relevant to the permisions that you have on the ftp server for the file to be deleted.