As per the current implementation of the RadGrid control, if we set the CurrentFilterFunction, that will be used for that column and even with no value, the same filter function will be used (specified in the CurrentFilterFunction).

This is not user friendly and one should expect that "NoFilter" will be invoked when there is no value.

There is no possibility for invoking the "NoFilter" command when the following properties are set:
ShowFilterIcon = False and AutoPostBackOnFilter = True

please refer the following link for more information:
http://www.telerik.com/forums/filter-not-resetting-if-filtervalue-is-empty-in-client-side-filtering