Form Extension - cannot remove field from grid

I am trying to remove a column from the grid using form extension from here

I would like to remove ln ty from the default grid so that users cannot alter it when entering orders but there is no option to remove any of the columns that are already in the grid. I tried customizing the grid, sharing, then completing the form extension but the grid just defaults back to the original even though I have the customized grid set to default. I wanted to do it via form extension so that the default form did not contain the ln typ. Any suggestions?