If you are facing attrition penalties after a contract is signed your options are much narrower but there are a few things you can do.

1. If you have a last sell clause in your contract, make the hotel confirm occupancies on the nights in question. They can do this with an internal flash report or occupancy report. Last sell means just that - your rooms will be the last rooms sold. So, if the hotel had 10 empty rooms on a given night and you exceed attrition by 10 rooms, you will be responsible for all 10. If they only had eight empty rooms, then you are responsible for eight. Also, verify that there are no other groups in house over the same dates that have a last sell clause in their contract and are also facing attrition penalties. If this turns out to be the case, then find out how the hotel is handling that situation. They should not be collecting from more than one group on a last sell basis if they only had 20 empty rooms but group attrition equal to 30. They should only collect for 20 and you need to know what number of rooms is being allocated to each group.
2. This one is fairly complex and a lot more time consuming. If you picked up 200 rooms on a given night but had 250 people register for your conference and pick-up credentials at your registration desk, then they obviously are staying somewhere. This is the time to ask the hotel to match their in-house guest list against your registration list. The odds are good that you will find some attendees staying in-house that made reservations outside the block and did not use the reservation method and group code you distributed. Any such rooms that are found should be credited to your block no matter how the reservation was made.
3. This tactic won't alleviate the penalty but it can make your life easier. Most organizations have a good idea about any attrition penalties 30 days in advance. These penalties can be applied to the upcoming conference any number of ways. Say you have $15,000 in attrition penalties. Tell the hotel you are aware of the penalties and would like to upgrade your F&B functions by $15,000. Maybe it's another course, lobster instead of fish or just better wines. Or maybe you'd like to upgrade everyone to suites - it doesn't matter - just spend it at the hotel during the conference. Not only will this make your attendees happy but the $15,000 will show up as a budget overage as opposed to a penalty on your master account. Experience teaches that most planners would rather explain an overage than a penalty. The hotel is going to get the money anyway so why not make the most of it? Don't expect it to work every time but it is certainly worth trying.
4. Ask the hotel to apply any penalties to a future meeting. Hotels always want to rebook business and to maintain client relationships. You will have to pay the penalties as outlined in the contract but the hotel may be willing to consider those penalties (or a portion thereof) as a cash deposit on a future meeting if it is booked within a certain period of time - usually a year. If you are willing to sign a contract immediately, the penalty may be waived entirely. In rare cases with chain properties, they may make some type of allowance as long as you book something into a chain affiliated property.
5. Simply offer to settle with the hotel. Some organizations do not have the budgets or financial means to support large scale attrition penalties. Be truthful with the hotel and let them know what you can afford to pay. You might be surprised at the result. Hotels are generally not interested in taking legal action unless they feel they have no other option and the amount involved is substantial.
6. If you know you are going to be in an attrition penalty situation, take it upon yourself to try and resell the rooms. It is not easy to do but it can be done. You might have to do it at a substantial discount but if successful, it will help mitigate your loss. Let other divisions in your organization know about your situation. Use your business contacts and your membership in any professional organizations to spread the word. Make sure the hotel knows you are trying to help resell the rooms, especially if you don't have a resell clause.