From here :
"Please note that all data will be deleted from the HTC Chacha as a result of this procedure. This process may or may not delete data stored on the microSD card. We recommend backing up all of your personal data before proceeding.
First, try this:
From the main home screen, tap menu
Tap Settings, then Privacy
Tap Factory data reset
Tap Reset phone
Enter your password if required, then tap Erase everything to confirm
If that doesn't work, or if your phone is frozen:
Turn the power off. If your HTC Chacha is frozen, pull the battery out and reinsert it
Hold the Volume Down button
Press and hold the Power button until the device turns on
You are now presented with a menu that allows for Fastbook, Recovery, Clear Storage, and Simlock
Select Clear Storage by pressing the Volume Down button
Press and release the Power button
Now simply confirm your decision: Volume Up for YES and Volume Down for NO