Sponge tool 
is located in the bottom right corner of the board and is used to remove cards from timetable.
Using sponge tool is simple; just click sponge tool (or press [Ctrl + Backspace] key) to select it and then click card(s) you want to remove. Hold mouse left button to remove more cards.
How to remove the whole timetable or specific day?
With sponge tool selected, click on day or period (in timetable header) to remove the whole day/period or click on the first column in the timetable row to remove the whole row.
There are also keyboard shortcuts: Shift key affects the whole timetable, Control key affects the row and Alt key affects the current column. You can do the same even without using sponge tool, e.g. click on sponge tool with Shift key pressed and all timetable cards will be removed.
To drop sponge tool just click tool once again or click on an empty timetable period or use Esc key.
As always, undo 
and redo 
can be used to revert or to redo the last timetable change.