Hi Matt,
Thank you for your interest in Syncfusion Products.
If you are trying to use a foreign key lookup column and you wants to filter it, then currently you have to use a foreignkey relation (and not just a combobox).
Please refer the sample to use a foreignkey lookup not showing the custom value in the dropdown :
http://websamples.syncfusion.com/samples/Grid.Windows/Grid_WF_GGCCustomRemove/main.htmPlease let me know if this helps.
Regards,
Jisha