Whatever we decide upon, I suggest a ban on VICIOUS rewards as preps, and on prepping Reflex, Quicksilver and Whoopaz
The problem is that this is impossible to enforce. I think that we just need to build it into the scoring presumption for the purist medal and leave it at that. Besides, having a slightly over-valued purist medal is probably a good thing for beginners, since they don't need to worry about bleeding gold from their treasury to participate.
Relying on honor to enforce this might look like too much, but if the vets are playing the same dungeons as the casual/mid/newbie people, we can at least know when to raise our eyebrows and inquire as to how someone managed to pull something off when it looks like your or avatars recent victory screens.
While this is true, it becomes very difficult to enforce the honour system across a wider range of skill levels. A moderate-level player who "cheats" with banned preps/items isn't going to raise too many eyebrows.
The only way I feel we can really move to anything more advanced is with the support of the devs. Something similar to the "high score" system in the alpha where players can submit runs and we can verify medals, time, kills, and preparations (perhaps even glyph/subdungeon/boss/shop spawns).