Accesing blocks from the server via FTP
First of all, blocks are stored in the Database, not in your web directory. Therefore, FTP is useless here. 

If the custom block was created using a custom module, simply disable the module.

If the custom block was created using the Drupal User Interface, you can disable your custom block by changing the status from 1 to 0 or edit the block body by login in into your Database using PHPMyAdmin and finding the correct block table.