Stop users from circumventing data validation rules

[munjalsavla]

I have a problem where users can circumvent data validation rules using certain tricks explained below. How do i restrict users from doing so.

I have a form where the user selects and item from a referenced dropdown and inputs the quantity. Only for a particular item can the quantity be greater than 1. For all other items, the quantity can only be one. 

I have set the validation rule as such: 

not(and(if([_Thisrow].[Item]<>"Apple",True,False),[Quantity]>1))

This validation fails when the user performs actions in this sequence:

1. Select item whose quantity can be > 1
2. Change quantity to a higher number, for example: 3
3. Now select another item whose quantity cannot be > 1
4. Saves the form.