http://i.imgur.com/XzvMsBW.png <-- is the button you are looking for. Under https://nerd.nu/forum/index.php?app=core&module=usercp&tab=core
And if she is still harassing you can add her to the ignore list under https://nerd.nu/forum/index.php?app=core&module=usercp&tab=core&area=ignoredusers
EDIT: And sent her a warning telling her to knock it off.
@c45y if there was a setting to make it the default I'd have had it on. But there is not from what I can tell.