One of the most common special offers is the non-refundable rate: this is a lower rate than the standard rate, but it will be impossible for the customer to request a cancellation with a refund, no matter how much time is left before the start of the booking.
As with any other special offer, you will need to go to the dedicated tab and create one.
Once you have done this, you will find the various parameters that you can modify as you wish, but the most important thing will be to set the fact that the booking is non-cancellable.
Finally, you'll need to set prices that are lower than your standard prices.