You could probably do it by making that an add to cart button and just make the quantity field hidden and set to "0" with the "If item is already in cart" : "Replace" option. I've never done it, but it seems like that would work.
Another option would be to add an ecart editable field to the row for quantity and again set it to zero and make it hidden and use the "Update Cart" server behavior. That is probably the way I'd go to get it done. Then you could have a separate form for each button and each button with the same name so that you only need one "Update Cart" server behavior to cover all of them.
You would have to get the WAUID value from the associated cart item. How are you identifying when an item is in the cart or not? I think you could use the value of: $cartObject->GetInfo("ID","WAUID")
so if you just added this to your form:
<input type="hidden" name="name="eCart1_Quantity_<?php echo $eCart1->GetInfo($row_recordset['ProductID'],"WAUID"); ?>" value="0">
That would work if the Update Cart server behavior was triggered when that form was submitted.