Hi Sunil,
Thank you for writing,
RadComboBox does not have a built-in functionality for turning of the auto-complete mode. You can, however, set the AutoCompleteMode to None on a button click or on combinations of keys pressed while RadComboBox is focused.
All the best,
Peter
the Telerik team
Check out the Q1 2011
Roadmap for Telerik Controls for Windows Forms.