Prevent block from caching
Drupal caches the already rendered block and doesn't check for changes in code, so after changing #cache in the block's render array as shown, you'd still need to use drush or the UI option to force-clear the render cache after changing this.

As an alternative, try deleting the block instance and placing it again.

Since you don't indicate that the form is making changes to the database, it's not clear if there is any cache tag to associate - is it data input or something more like a search form?