org-agenda-block-separator

org-agenda-block-separator is a variable defined in `org-agenda.el'.
Its value is

61

Documentation:
The separator between blocks in the agenda.
If this is a string, it will be used as the separator, with a newline added.
If it is a character, it will be repeated to fill the window width.
If nil the separator is disabled. In `org-agenda-custom-commands' this
addresses the separator between the current and the previous block.

You can customize this variable.