Drop date in dynamic dated class

How do you enforce the last date to drop in a dynamic dated class section?   The date in Class Section Dynamic Dates doesn't appear to stop students from dropping.  The date in Session Time periods (End of Drop/Add)  can grant self service access to drop and or add, but it apparently is just defining the transaction period and not the final drop date.   This is important to us because our last drop date and last add date are different.   What am I missing?