A nick only blacklist is sufficient since only registered nicks can send memos. Lets keep that part simple

As far as registering alot of nicks to abuse memoserv .. you can already limit the number of nicks registered per email. Yes, people can have multiple emails .. but it's just as easy to have multiple IP's/proxies. Using proxies/bouncing is no where as "1337" as it used to be. Not to mention that MemoServ only sees nicks at this time and not hostmasks. To make MemoServ see hosts would require a core code change as well as a db change.
Also, this module should honour the whole group .. so that those with 10 nicks don't have to be blocked 10 times.