Hi Ivor
There is no payment method or Lock on the documents level.
Also note that if the Security deposit refund is the only open item in CA , it does not get dunned. It is only when there is open debit item along with Security deposit refund that the open credit gets dunned along with Debit. Ideally the open debit & credit should be balanced in clearing run but for some docs this does'nt seem to happen leading to this issue.
I have a separate discussion open on auto clearing
I will try the debug approach suggested by you by getting one of our ABAP guys involved.
Thanks & regards
Satish