Yes, there will most certainly be. Two considerations come to my mind:
- you cannot cancel the last day only, a cancellation will cause the entire reservation to be foregone and even if the hotel were to agree to let you stay for the reduced number of days spanning the conference only, that would be at the walk-in best flexible rate, would would likely be prohibitively high
- if you want to leave before your reservation ends, the hotel will apply an early check-out fee, make yourself acquainted how much this is, given the situation you are describing it may as well be higher than the cost of spending an additional night

None of us can tell you for sure. It is possible that the hotel agreed with a third party on a block reservation for a large share of its capacity and thus nothing shows as free. If the third party committed to pay the block in bulk, then that capacity will never be released for outside-of-the-block bookings. Alternatively, if the block has an expiration date, the capacity will re-enter the reservation pool after the expiry has materialised. Another possibility is that the hotel has imposed a minimum length of stay that is longer than the duration of the conference. But I am purely speculating here (based on my own past experience) - if this is so important to you, contact the hotel directly and inquire.