Retrieving real content that's been deleted by spammers is always possible on a Wikimedia wiki, because it tracks the complete history of edits on each page. Basically, if you find yourself looking at a page that says just some nonsense, where you would have expected real content, you click on the "History" tab and look for the last edit by someone who was really working on the article, not a vandal. You then click into the version of the article you want to return to, hit "edit" for that version, and save. That will overwrite the current version of the article with the old version.
Alternatively, if there's just one edit that was made that was vandalism, you can click "undo", which is a link at the far right of each entry in the page's history list, then save the page.
Interpreting the history list is easier when real people register recognizable usernames and always log in to edit. What can also help though is often the spambots leave an editing comment that's complete gibberish, like yKtdjhtRb. Making detailed Summary comments when you edit as to what changes you made can also make it easier to distinguish between real edits and vandalism. If you're not sure about an entry in the history list though, you can easily compare it to the previous entry. Just click on the radio buttons in front of each entry then hit the "compare selected revisions" button at the top to see the changes highlighted.
For instance, yesterday I noticed the K. Tempest Bradford article in the Carl Brandon wiki had effectively been erased by vandals. When I looked at the history, it looked like this (plus two columns of radio buttons, which run between (prev) and the date/time stamp):
(cur) (prev) 14:55, 13 November 2011 126.96.36.199 (Talk) (46 bytes) (rRHjrefbzPRVc) (undo) (cur) (prev) 08:04, 13 November 2011 188.8.131.52 (Talk) (59 bytes) (PzlFqAZYKGfVNrcgy) (undo) (cur) (prev) 07:40, 13 November 2011 184.108.40.206 (Talk) (2,184 bytes) (→Editorial Positions) (undo) (cur) (prev) 08:07, 29 October 2011 220.127.116.11 (Talk) (2,335 bytes) (→Projects) (undo) (cur) (prev) 05:17, 18 July 2011 18.104.22.168 (Talk) (2,427 bytes) (undo) (cur) (prev) 17:06, 9 October 2009 WendyDawson (Talk | contribs) (2,424 bytes) (→External Sources) (undo) (cur) (prev) 09:12, 14 September 2009 Zeborah (Talk | contribs) (2,268 bytes) (→Works: link to Enmity) (undo) (cur) (prev) 18:27, 27 July 2009 Sparkymonster (Talk | contribs) m (2,217 bytes) (undo)
The changes made that day were obviously spam, but I wasn't sure about the previous two. Using the "compare selected revisions" function I determined that the July edit was just a category change, but everything after it was spam. So I clicked on the date, 05:17, 18 July 2011, then clicked the edit tab. I see a warning, "Warning: You are editing an out-of-date revision of this page. If you save it, any changes made since this revision will be lost." Perfect. The changes made since that revision were all spam. I entered "reverting to last complete version" in the Summary comment box at the bottom, checked "Watch this page", and hit Save Page.
It takes a little bit of time, but it's pretty easy. If more people could watch and patrol pages, wikis like the Carl Brandon Wiki, the SF Editors Wiki and the SF Artists Wiki have a much better change of survival. I think these could be valuable resources for the community, so I hope people will.