I've gotten tired of seeing newsgroup claims along the lines of
"since devteam is aware of this and has chosen not to eliminate it, they
must endorse it", so weaken the tactic of "pudding farming". It is still
possible to gain unlimited experience (past level 15 or so there's not
much point), but will be less effective for gaining items and for providing
sacrifice fodder. Keep track of which monsters have been created via
cloning (mostly puddings; gremlins and blue jellies are affected too but
nobody's likely to care much about them) so that they can receive special
handling. Make cloned monsters progressively less likely to leave corpses
as the number killed for a particular type goes up, and also much less
likely to drop random items at death. This is sure to need some tuning
once hard core farmers point out how they can still abuse it. For the
absurdly extreme case, see
http://scavenger.homeip.net/farmbot/HomePage
FYI, farmbot/PuddingFarmingHOWTO includes an impressive screen shot of a
dungeon level where rampant farming is taking place.