Maintenence and support of wakka seems to have ceased.
There are no anti-spam features, and seemingly nowhere to go for help with developing them.
It doesn't have EditSummaries, which makes it difficult for users to despam. But even if they do, the spam is still indexed (see NoIndexHistory)
Do not install this software. Not sure why anyone would pick this engine to install, or where they could download it from, but some fresh installations have continued to appear.
If you are running a wakka wiki, migrate to a better engine! You could Migrate to WackoWiki or Wikka. Since they have an active development / support community. There are some patches described on the wikka website which might work on your wakka installation (try it out, but keep a backup!). See WackoWiki Wikka
See also AntiSpamRecommendations