Item Substitution- Planning Details - for all but 7 customers

Hello Gurus,

I need to setup substitution for some items for the period of a couple of months for all but several customers. Is it achievable using Items Relationships - Substitution - Planning details form?
We have hundreds of customers and only few will not take substitution. Do I need to list all but these customers? Will it work if I created substitution for all customers and after that substitute it back to original item for some customers.

Example: Item A will be substituted to item B for all customers
               Item B will substitute back to item A for some customers