What we’ve done is hide all the Reply buttons in Wiki Categories so people can’t make replies. Doesn’t prevent reply-by-email, but the creation of new wiki topics is rare enough for this not to be an issue.
// Hide controls in Wiki categories to prevent new replies
.category-tools .create {
display:none;
}
// Ah but make sure you can save edits!
.category-tools .submit-panel .create {
display:inline;
}