I found that every page and discussion page in my Society and Environment Development wiki had been written to anonymously with spam links. Also most of my content had been deleted. Fortunately there are only about 6 pages but it still took me an hour to work out what to do and get rid of the mess. Basically, for each page (article and discussion) I had to revert to the last version before the spam was added, delete the page and then restore the page selecting only the revisions I wanted to keep.
Sadly I have also had to change the permissions so that only logged in users can edit.
