I can't really imagine that actually helping people who know the /spamfilter command as it is, but you could make it helpful by allowing straight text for the reason and then using something like:
$replace($did(27),_,_,$chr(32),)
It would also be more helpful if those checkboxes were words, rather than the actual character used in /spamfilter, e.g. 'Private Messages' instead of 'p'.
Good job with the script, though. :)

By the way, Unreal supports the 'warn' action now.