Restrict access to certain actions in certain areas?

Solution 1:

Not sure if this will help, but check out this video..

The above link essentially tells you to detect a dropped item, kill it, and then replace it.

If that didn't work, I remember an old solution, but I'm not sure if it still works. Using the PickUpDelay tag, you can make it so players instantly get their items back. I think running this on a fast clock may work. WARNING I'm not sure if it works and it may lag you out.

/execute @e[type=Item,r=5] ~ ~ ~ /entitydata @e[type=Item,r=5] {PickupDelay:0s,Fire:-1s,Age:0s}

The second tags inside of the curly brackets seem to need to be included for the command to work successfully

If the above command does not work, try changing the "0" to something else.