A supplier (SUP) account can be closed/discontinued using the option: 'Close Supplier (St=9)' if you have access to this option.
If the supplier has open/active documents or posts, it will not be possible to close/discontinue the account. In that case, a warning message will appear.
To see if there are open documents the following restrictions can be used to help identify open posts:
ITM;ACCOUNT:xxxxxx,ST1<"9"
PAY;ACCOUNT:xxxxxx
(xxxxxx = supplier account identification number)