RadComboBox supports autocomplete functionality - you can set a poll-server timeout and the combobox fires a server-side event that returns all the relative matches for the currently typed text. You can easily select an entry or re-request more entries depending and the ShowMoreResultsBox property. This example shows autocomplete from SQL.

When you load the combobox items on demand (in the ItemsRequested event) the AllowCustomText property is enabled by default. You cannot disable it as this would break the load-on-demand mechanism.
The ItemsRequested event is fired when the user types in the Telerik RadComboBox input field or clicks the drop-arrow image. Disabling the AllowCustomText property would prevent the user from typing in the input field and this would break the load-on-demand mechanism. Therefore, we decided that the best approach is to hard-code the value of the AllowCustomText property and set it to true when the combobox is in a callback mode.