You can find the fields for setting a close down date and time in the "Changes" tab in the Event Profile, under the "Close Down Dates" sub-heading. From here, set what date and time you want close down to go into effect. Note that the hour drop down is written in a 24-hour clock format, meaning for example "Hour 17" means 5:00 PM, and "Hour 0" means 12:00 AM.

Keep in mind that if you are signed into the MCP when submitting a change request any time after the close down date, you will still be able to submit requests, meaning the fields to edit the reservation details in the change request process will not be greyed out - fully accessible to you. Don't get confused! :)