looking at the wiki for the software I found a section talking about Different Rights for anonymous users, signed in users etc.., it then explained what the various categories of signed in user were and what they could do.
Are you not the highest level i.e. Developer/Admin? If not I think you might need to be.
At the bottom of this list it said Other permission schemes (e.g. only signed in users can edit pages) can be configured.
So the possibility is there. I'll rummage in the user forums and see what they say about doing that and what level of user you need to be to do it. I also looked at Special Pages in the Wiki. There was one near the bottom about dealing with user rights. You can apparently deal with groups. There doesn't seem to be an anonymous / non-logged in group. I'm not sure that it would make a difference if there was. I think, at first sight, that these groups are just filters to make looking for users a bit easier.
Obviously I need to look harder. I suspect you need to be a developer to twiddle with these permission schemes. I did say I thought it might be something you do.
I will keep looking but not much more tonight, I've got to run a dance class tomorrow and I'm still in sleep deficit from starting out with TitchyLinux this time last week. I'm also going to have a bit of a social life tomorrow evening, so I won't be on until midnight my time!
I'm slightly puzzled by the fact that there is so little spam activity in the Recent Pages.
Am I right in thinking that the link to the wiki is slightly different, in the same way as the forums? If so perhaps the spambots are confused!
Perhaps all these bots were coming from a fairly small range of ips and the side effect of those bans I've put in have had an effect? We'll find out.