Using Excel 2007 as roster...

Hey guys,
In the past I've had a lot of help from you guys in building this spreadsheet.
Right now I want to add an "Unavailabilities" section to help me in rostering.
I have inserted a new slide marked "Availabilities". I want this to be the basis for rostering. At present each employee are available for all the time, but I want the 1st sheet "Roster" to be able to interact with that sheet.
For example, If No. 1 is unavailble on Monday's after 5, I would like Excel to warn me/stop me if I was to try rostering this person after 5.
If you can work out a way to do this I would greatly appreciate it.
Also, anyone that has any suggestion's on the improvement of this worksheet feel free. I have been working on this for a number of months now and so far am very happy with the product so far but there is always room for improvement!
Thanks in advance,
Regan