In Death Zone scenario, does  not allow other characters to use Toxic Clouds, etc. (Like does not allow to roll die at all ). 

Did you mean that you encountered Scarlet Walker and some other character could not play Toxic Cloud on its check? That is as it should be - read the Scarlet Walker and it says "Immune to Poison". Toxic Cloud adds Poison trait, so it may not be used on this monster (and all undead).

Likewise, if you encounter the Leng Spider and the 1d6 rolls a low number, you may not play Attack spells. That includes even Web, Toxic Cloud and other supporting non-Aid spells.

Are you also talking about Leng Spiders? cause those have Diplomacy check as default and you have to change it to combat before being able to use spells with attack descriptor.

Scarlet Walker is immune to poison and a few other elements as Jenceslav pointed.

There are however bugs like that. Hidden beast for instance has a combat check before you act that if you fail you bury cards. On this first check you can use weapons and spells with attack descriptor. However, on the actual combat check to defeat it you can't use weapons or attack spells. Go figure...

Nah, this happens to me a lot. For example, if Kyra is going against a Hag, there is no way that Lem can play a Disintegrate or a Gozreh's Trident etc etc 

Vinceicarus, when someone fights a monster, other heroes cannot play most Attack spells, so this is as-intended. The main exception is Swipe (helping spell -3 difficulty) and Sunburst (the same). I do not have Gozreh's trident, so I cannot attest to that

The trident is a attack spell.