Problem blocking users in Drupal 6
Do it in the actual database, not via Drupal but via CLI, PHPMyAdmin, or whatever you have.


Find out the user's ID. 
Go in to the raw database, i.e. not via Drupal but via CLI, PHPMyAdmin.
Go the user table.
Find their ID.
Set the status field for that user to 0 (which means blocked).


The raw SQL would be something like (I haven't tested this):

UPDATE users SET status = 0  WHERE id = [user's id]