Subodh,
You have a few options.
a. To disable call forwarding from the phone itself, you can set a CSS which has no access to internal phone partitions nor route patterns to dial outside.
b. To disable users access to call-forwarding from the web user interface, you can disable this setting from Enterprise parameters. The next time users log in he wont see that option show up on the webpage. This is a global setting.
c. You can optionally set a custom softkey template and remove Cfwdall option in the Phone Idle state and apply that softkey template to all phones. This will totally remove the Cfwdall option from the phone.
d. Combine options b and c you will completely deny access to any user/phone to call forward his phone to another destination number.
HTH
Sankar
PS: please remember to rate posts~!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus