If so, yes, they do and your enemy will be at 0 CR. Although 75 is the max EFFECTIVE elemental resist (unless you have +to max resist), the total is unaffected. You cannot have lower than -100 resist, but I don't believe there is a limit to how high you can stack.
+Add up all resist bonuses from items, skills, auras, Anya scrolls...
-Subtract all penalties (difficulty penalty for nightmare/hell, auras, abilities, etc)
=75(or whatever the max is) or the result, whichever is lower.