Each opponent can't cast non-creature spells with converted mana cost greater than the number of lands that player controls.
Whenever an opponent casts a spell, if no mana was spent to cast it, counter that spell.
"I told Jace that Ravnica would fall apart if he didn't take his responsibilities seriously. I didn't want to be right."
illus. Steven Belledin # 189/259
Quote from Mallwan »Deos 0 mana mean the same as no mana? There are cards that specifically say "cast without paying its mana cost" and on the other hand, cards that cost 0 mana are cast normally (i think by paying 0 mana) . I'm not sure here if that creature here stops 0 mana Artifacts.
Quote from Kenaron316 »Moxes cost 0, and you're not allowed to pay more than that, so this gets those. For convoke, you can just pay the one actual mana to get around it. Affinity once it hits 0 that's it.