Sharply cut the frequency of lizards spawning (possibly even lower than 1/7), and possibly remove the special case that makes them always leave a corpse. This is because a player will usually accumulate enough of their corpses to be easily safe from any delayed stoning effect for the whole game. This won’t much affect reverse genociding them, if the player really wants a lizard corpse.

Also, possibly elevate their monster difficulty to something higher than 6; not because they are that difficult, but to make them appear only later on in the dungeon.

Alternatively, simply make lizard corpses rot, so that a player who wants to carry around a stoning defense must use other items, such as tins of lizard or acidic monsters, or potions of acid.