Changing a drop down list: Is it possible to use GuiXT in order to exclude certain document types in an SAP drop down entry field?

No, this is not possible, but here is a workaround. Delete the standard drop down entry field and create your own, using the "DropDownList" keyword. You can fill the drop down list with a fixed number of document types, or you can use a "Call" to read all SAP document types and then exclude some of them. For details please see the keyword documentation for "DropDownList".