User blog comment:Dopp/Message Wall Now Available in Labs/@comment-2208586-20120203124827/@comment-1032852-20120203131801

You could apply the following to Special:MyPage/wikia.css or Special:MyPage/global.css. It doesn't completely remove message wall posts from Special:WikiActivity (though this could be done), but it makes them much less noticeable. table.wallfeed {display:none;}