removing valid values from system form combobox

Hi all

I have a User field which i added with valid values to a system table.

when i click on the combo box of the relevant field in the System form which relates to the system table to which the user field was added. i wish to delete the valid values of the combo box and insert new ones.

problem is when i call the Remove method on the Combobx object i receive the following message:"Item - The item is not a user defined item".

question is how can i remove and add valid values to the combo box user field?

i work on SBO 2005 A sp01 DI version 6.80.318 PL 14

appreciate the help

Yoav