Whenever a player casts a spell from their hand, Knowledge Pool triggers, exiling that spell.
Because Eidolon of Rhetoric prevents players from casting more than one spell each turn, players cannot cast the spell permitted by Knowledge Pool, or any other spell that turn

Exile all spells cast from a player's hand

Players can only cast one spell per turn

Knowledge Pool

Eidolon of Rhetoric

Any time a player would cast a spell, Possibility Storm exiles that spell and reveals cards until you find one they could cast.
Eidolon of Rhetoric prevents them from casting a second spell resulting in a lock.

Combos with "Eidolon of Rhetoric"