Is there a way to suppress referential integrity for specific records?

Question

I have a table of people and another deliverers holding details of how deliveries are made. Most deliveries are made by someone in the people table so there is a peopleId which points to the people table. However, some are generic e.g. post, collect, etc. i.e. there is no peopleId. Is there a way of e.g. leaving peopleId blank or null which would not cause a referential integrity problem. The only other way I can think of is to add dummy records to the people table.