Revoking an assignment at the IANA-to-RIR level would be an extreme
step; I'd expect it to be taken only when other measures have failed.
But it seems to me that they *have* failed; there is rampant
irresponsibility - or at least behaviour that looks like that to me -
at the RIR level. At least two RIRs don't - or historically
haven't; I
haven't checked recently - enforce contactability of address block
holders, making it easy for abusers to get space and duck any
responsibility for its use. RIPE, at least, flatly refuses - or at
least has refused - to take any action against address block assignees
who don't police their assignments; I haven't had occasion to push it
as high as the RIR with other RIRs, so I don't know how they are in
this regard. (The abuse I see most often is spamming and spam
support,
but I feel quite sure that's an artifact of my perspective more than
anything else.)