Mending, cause it's not the REAL solution.
24hs, because IPs change.
"At least it will reduce the amount..." Yeah, I know IPs change and they can use proxies... It will only slow them down, thus reducing amount of spam zone messages.
I didn't read of anyone getting banned by mistake due to the filters banneing this chars, so it's probably a good filter
Another simple measure is to have a delete char timer, since they are probably deleting chars and creating new ones this would force them to create new accounts which will slow them down.
Also the simplest one is to have to do a quest to enable zone chat (as opposed to setting a lvl requirement), that way they can't use the char as soon as created, they will have to at least bot it till the quest is done, which slows them down and makes them easier to pinpoint.
--- Good criticism includes counter ideas.