Only Admins can ban users from editing Wiki pages - see the Wiki page at Special:BlockList for the current list. The problem is that the users in your image who posted the SPAM are all using throwaway usernames, almost certainly created just for posting the SPAM so on their next visit they will be using a different username.
And no, I would not recommend restricting posting rights to only certain "favoured" users, otherwise I would never have created my first Wiki page.
But thank you for pointing out this problem. I have far too many Wiki pages to spend my time constantly checking them for unauthorised alterations (SPAM). I suppose I could just tick the little box on each page that says "Watch this Page".
I've been cleaning the odd page if you or RailPilgrim hadn't already done so. I believed only N3V folk can actually erase those pages. Zec is aware of this issue.
