Nothing to do with being shunned. The STMP is a FAA system for managing traffic during special events. At times when the amount of traffic cannot be reasonably predicted, the STMP system allows the FAA to control the amount of traffic to be handled, by issuing slots based on a reservation system. The FAA computers don't store flight plans longer than 2 hours, and the slot revervation system allows reservations and arragements to be made longer in advance, which facilitates planning and coordination.
If you show up and don't have a slot, then you might have a chance of landing if someone didn't make their slot and there's an opening. Otherwise, you won't be given the option of landing. You'll be turned away. You may also have problems departing if you haven't made prior arrangements.
The NOTAMs published for STMP operations will specifiy w(h)eather the program in effect for a given time applies to IFR or VFR, or both, and w(h)eather it applies to arrivals, departures, or both.
If you arrive without a slot, you'll be asked your intentions, but you may not have the option of getting in. If you don't have fuel to go elsewhere, then you'll probably be faced with using emergency authority, which could place your pilot certificate in jeopardy (for creating the emergency).
Even when a NOTAM indicates that slots aren't available, it will include contact numbers that you can use to find out about cancellations, or additional openings that aren't showing on the ESTMP system. However, attempting to get in by flying VFR in the hopes of bypassing the system and violating the conditions set forth is asking for trouble.