Looking at 197548b5, the relevant commit for SMACK-346, reveales that it was at first forbidden, but then got changed to no(t)_acceptable. XEP-95 4.2 specifies forbidden, this was obvisouly what the inital author of the code had in mind. XEP-65 5.3.1 Example 13 mentiones not-acceptable.
I tend to favor not_acceptable right now. But this should be discussed first.