I have misunderstood the issue, please excuse me about that.
There is no build-in configuration for customizing the columns in the Gantt assignments editor. To achieve the desired appearance, I would suggest you to subscribe to the edit event
of the widget. In the edit event handler you can get a reference of the 'Assign' button. Note, that you may need to use setTimeout
function in order to ensure that the button is loaded by the time you are trying to get a reference. Once the button is clicked, you could get a reference of the Grid, that is in the assignments editor. Next, you can hide the column, by using Grid`s hideColumn
Here is a Dojo example
button = $(
resGrid = $(
where the described approach is implemented.