Sorry small correction to my initial post.
Even though you removed the credit group for the respective sales order type still system update the values subject to if the respective item category the "field credit active" active.
Removing the credit group to sales order type controls whether you want to perform the credit check or not. And your requirement is not to update the particular sales order document values in FD32 so the actual control is credit active field for that particular item category.
Since you have mentioned that you don't want to update that particular sales order document values( assuming that you are taking about receivable only).
Please note sales order determines the item category, so do as below.
De select the credit active field in the item category which is determined based on the sales order type for which you don't want to update credit values.
thanks,
Srinu.