Samsung T959 Galaxy S T-Mobile Vibrant hard resetTry this first, maybe you have to try it more times, but it shuld work.
1. Power off Galaxy S Vibrant.
2. Press and hold both Volume buttons + POWER.
3. Press these 3 keys until logo appears, then release them.
4. After a few seconds you should see factory menu.
5. Select "delete all user data" with volume button.
6. Press POWER.
7. Scroll down to Yes - delete all user data and press POWER.
8. Select Reboot system now!
All data will be lost!
PRESS AND HOLD VOLUME UP + Home Button
Keep pressing these 2 buttons + press Power button for 2-3 Seconds
Release only power button, but KEEP pressing Volume up + Home Buttons
With Volume down select wipe data/factory reset (3rd) > Press Power to select.
Source: original link see http://www.hard-reset.com/samsung-i9000-galaxy-s-hard-reset.html
T959 Factory Data Reset via menu
From this menu you can reset your phone and sound settings to the factory default settings.
1. From Idle mode, touch ➔ Settings ➔ Privacy.
2. Touch Factory data reset then touch Reset phone, enter your password, then select Yes to confirm.
The phone resets to the factory default settings automatically and when finished displays the Home
Use this method to hard reset your Samsung Vibrant Black phone: Enter *2767*3855#. Warning! All your data including contacts, messages etc. will be lost! Copy all your necessary data/Contacts/Messages etc. to SIM or make backup to your PC before full reset! Take out SIM card before full reset.
When your phone is frozen take out battery and reinsert it.
If you know the hard reset using buttons / keys, pls leave a comment. All comments are welcome! :)