Shouldn't this work?

   Items.TextField.ChoiceList.AvailableValues = BancosList.Copy();
   
      ShowedValues = Items.TextField.ChoiceList.AvailableValues;
      ToDelete = ShowedValues.FindByValue("89");
      ShowedValues.Delete(ToDelete);
      ToDelete = ShowedValues.FindByValue("16");
      ShowedValues.Delete(ToDelete);

Could you please provide a configuration where the problem can be reproduced?

Hi Tim, I met the problem. I need your helping. "I have a choicelist in the properties of Data processor. I want to set Source data for ChoiceList. How I do it work? ". Thank you a lot of. Nice too meet you.

phat tran, hello!
In 1C:Enterprise setting data source of choice list isn't like in, for example, MS Access or VBA userforms.
When you set reference data type (catalog, document) of attribute in your data processor, the control linked with this attribute will automatically get functionality of choice fr om list or object list form. No need to manually set data source.
If you want to lim it choice list, you may use the "Choice parameters" and "Choice parameters links" properties of form control.
Also you may fill the choice list programmatically.