Max, there is no mod that prevents building indies. The way you do it is add the following into the map file for each province
#setland <province nbr>
#poptype 100 (any value greater than 87 will do)
That will remove the independent poptype, but it will still get defenders, because poptype removal only happens after the defenders have been assigned. You have to do a separate no-indies version of each map you want to play that way.
Independent strength means just how powerful the defenders are (i.e. higher value = more of them).
As far as what indie mages are useful, the Enchantresses, Enchanters, Sorceresses and Metal Order mages (Iron, Silver, Golden) are great, as are wizards. Depends somewhat on what paths you have and how high you can get them with nationals. Moon Mages and Elludian Moon Mages are downright brutal with Evocation 7, just put a horde of them in the back, script Nether Darts and watch the nemy army vanish under a rain of dark power.
Sages can be recruited from 3 sites: Library, Citadel of the Loremasters (unique, rare) and The Council of Sages (unique, rare). I think there might be a fourth one, but I can't remember for certain.
Of the indie poptype mages, only the Amazons are really worth anything bar special circumstances, or the lizard shaman if you lack astral magic (such as EA Ulm).