Need plug in to delete user name, delete sig file, but keep all posts.

[Jah-Hools]

Some members contact me to remove all their threads and posts and their membership.

I don't want to lose all their contribution and I think my Terms of Use support keeping the posts.

But I want to lose the user name (perhaps something better than 'guest') and lose the signature file.

Any plug ins for this?

Any thoughts on this practice?

Thanks in advance,