Do the following
1. Turn the Samsung Galaxy tab 10.1 GT-P1500
2. Press and hold the Power button and the Volume Up button together until it displays the Android robot
3. A recovery menu should display after
4. Scroll to Wipe data/factory reset (not Wipe cache) and use the power button to confirm the selection
5. Scroll to 'yes' using the volume button and confirm with the power button
6. Scroll to reboot now when the wiping is completed and confirm with the power button
The Tab is now factory reset!