NEUTRALIZER GUN: When Forge hits an opposing character with a ranged combat attack, before damage is dealt, choose a power that character can use. That character can't use that power or any combat or team abilities until your next turn

If I am using Forge and hit Ventry for 2 damage and choose Impervious as the power to ignore, after he takes his two clicks does he get to use the Impervios that is on click 3 or do all uses of Impervious get ignored?

If you chose Impervious, he would not be able to use Impervious from any source until your next turn. And the power wouldn't be "ignored", just not used. Ignored is a specific effect in Clix.