Indeed you are right - in this case the edit/insert operation will be processed regardless whether you press the Cancel button or not due to the specific event sequence
when the viewstate of RadGrid is disabled.
The only option I can offer you is to remove the cancel button from the edit/insert form (thus disabling the cancel option) intercepting the ItemCreated
event of the control, namely:
sender, GridItemEventArgs e)
GridEditableItem && e.Item.IsInEditMode)
I hope this is feasible for your situation.
the Telerik team