They can be CC'd on Hardmode as well. When you reach the end-boss, it should be a full out burn on the boss, just assign 4 people to CC the turrets and leave them (you'll probably have to re-CC them once during the fight). Have your raid position on the cave-side of the boss, only your CCers need to be anywhere close to the turrets.
Completely wrong, you can NOT cc the 4 turrets that spawn in hard mode. You can do the two little dudes, but not the turrets.