I would create a column in the cart called MaxQty.
Then I would create a calculation:
TrueQty = (([Quantity]<[MaxQty])?[Quantity]:[MaxQty])
Finally I would Update the TotalWeight and TotalPrice Calculations to:
TotalPrice = (([Quantity]<[MaxQty])?[Quantity]:[MaxQty]) * [Price]
TotalWeight = (([Quantity]<[MaxQty])?[Quantity]:[MaxQty]) * [Weight]
Then wherever I display the quantity I would display the MaxQty instead.