The New Source Wizard does not allow the user to delete constraints after entering them. For instance, if I enter an IOStandard of LVCMOS33, I cannot delete the value.
Although the user cannot remove the constraints after he makes a selection, the dialog box shows up empty when a new line is being created. The empty selection is not available in the pull-down menu. To work around this, manually delete the constraint from the UCF using a text editor.