Go to Configure - AccessPoints. Press the "Show More" button if not all APs are shown.
Now use the checkboxes of the APs that you want to remove, and press the "Delete" button. This button is greyed out until you have selected some APs.
this is weird. ZD shall let you remove the AP from GUI. Please try a different browser like Firefox or chrome to be sure that it is not a cosmetic issue.
Is delete button for connected AP too greyed out?
Did you tick the AP who want to have deleted?