The Highest Feat Taken, either taken normally at level or given by whatever means (either applied to skin, an item, or directly to the character file), is what will always be in effect. So if the player had EER 4 and you took away EER 2, EER 4 would technically still be there and working (meaning they would still get 40 resist)...
EER does NOT stack with gear, whatever the highest resistance is, is what is used for resistance checks (e.g. subtracted from dmg. total before dmg. assignment)
Meaning if a player had, say Cold Resist 20 on gear, and took EER Cold 3, they would only get 30 resist instead of 50...
The only thing that stacks is Damage Reduction & Damage Immunity (which is capped at +/- 100%), DR (Dmg Reduction) is stackable in a sense, meaning you can have +10 / 20 DR & +8 / 20 DR, and if a monster with say a sword (up to +7) hit you with it, it would soak 40 dmg (20 two times), not just 20 alone.