(1) add the Geometry/Coordinate Editor control to the form
|Administrator - Form Designer - add control....|
(2) set form in Edit mode
(3) open Geometry editor (button will appear next to the Editor control)
|... Map - Form - use control to delete geometry.|
(4) In the Geometry Editor window you can delete - step by step - all vertices of a feature (you cannot delete the whole feature in one go). After deleting all vertices don't forget to "Update" the record to make the changes permanent.