Hi Geoffrey,
What I could get is, in the Dunning process, at one particular dunning level, you have configured the Collection Agency referral process. And based on the Business rules, all the items which reaches this level are being referred to the Collection Agency.
Now, the requirement is to dun all the items, but for certain transactions, you don't want them to refer to the collection agency, even if the Dunning will happen.
Going by the above assumption, first check the Dunning activity for the dunning level in SPRO, which triggers the Collection agency process and try to find the FM, which is triggering it.
SAP has given, FM,FKK_COLL_AGENCY_RELEASE_0350, for the Collection agency referral process through dunning.
You need to customize this FM and incorporate your own logic for excluding certain transactions (based on Main/sub, Document type etc.) and assign the custom FM to the Dunning activity.
Hope it helps..
Thanks,
Amlan