I'm not sure if your talking about your own forums and this is a feature you would like to see added to newbb in general, or if there is a problem here on the XOOPS forums themselves. If it's the former, there may not be much you can do except disable signatures and avatars. If indeed it is the latter case, then please read on.
Signatures and avatars are global. I understand your frustration, but it would be near impossible to lock a persons avatar or signature. The signature and Avatar are controled from the User Account, not the forum or post where they made their comments from. The forum is only a means for displaying those items.
When a forum is locked, the only thing that should (or can)be locked is the content of the posts. I do not know if a person can go in and edit a post once a thread is locked, but if one can, then maybe it's something that should be looked into a little further. If this is the case, please let us know.
Taking things out of context is easy and can usualy be fixed by the involved parties themselves. However it's a shame when it gets out of hand simply because of a misunderstanding. It's happened to me once or twice.
Cheap shots however should be taken with a grain of salt. If it needs to get that personal, then maybe it's time to move on to another post, or take it up in private. If the two (or more) disagreeing parties can not resolve their issue, then a moderator may have to step in. Petty bickering, although frowned upon, is an inevitabilty at times. We all need to excersise patience, myself included at times.
In any case, it is usualy encouraged that users take their personal matters to Private Messages. It's rarely a problem in this community where moderation needs to be strictly applied. The community at large is for the most part, fair and good to each other.
*Edit*
And realizing that your post was made in the module request section, I guess I ran off a little too much. A perfect example of something taken out of context
Sorry Chief!
(It's all good advice for others reading as well, so I'll keep my post going as is)