How to make a field NULL using MySQL GUI Tools?

In MySQL Workbench, with the cell selected, press Shift + Delete

Tested on 6.3.4.0

In MySQL Query Browser, right click on the cell and select 'Clear field content' while the focus is in another cell.

In MySQL Workbench, right click on the cell and select 'Set Field to NULL'.

Removing the contents:

This works for some tools (sorry to hear it doesn't for yours). This may not appear as null but will when you perform a query.