Hi Radek,
You can do this by deriving the GridTableFilterBarCellModel class and override FillWithChoices which is the method that fills the dropdown. Please refer the forum thread below for more information on this.
http://www.syncfusion.com/Support/Forums/message.aspx?MessageID=25998Also, check the following KB article for ore details.
http://www.syncfusion.com/support/kb/666/Sample mentioned in the KB can be obtained from the following link:
http://help.syncfusion.com/samples/kb/grid.windows/GGCRemoveCustom/Custom.zipPlease let me know if this helps.
Regards,
Jisha