To disable this block if your site is unusable;
Log on to your mysql drupal database.
Go to the "blocks" table and find the offending block.
Change the "status" field of that block to '0' and clear the cache tables.
You could also remove the code from the corresponding boxes table (bid) or set the block input filter to html instead of php (format filter 1 or 2 in the boxes table).
If your site is broken but you can still log in and make changes;
Log in and disable, delete or modify the block that breaks your site.