If you dont want to sell a single night (and this is a common issue with Saturday Nights in the Leisure Break Market) in the Inventory/Rooms section you can select the relevant day from the dropdown 'No single booking on...' etc etc.
The downside of doing this is that it prevents single night bookings on that night of the week for every week during the year (frequently hotels dont want to use this feature in Winter ie they will take a single night booking in December).
The only other option is to remove availability of rooms altogether for the night or nights in question, and instead make a 2/3 night package available. However remember that ONLINE 82% of sales are for rooms, NOT packages.