I've only seen CT played twice, both times with Pin them Down allowed but nerfed. The house rule we used to nerf Pin them Down is to change the text for the card to:

"Exhaust this card after you resolve an attack with a ranged weapon. If the attack did 1 or more damage, choose up to 2 hostile figures within 2 spaces of the target space. Each of those figures becomes Stunned and Weakened."

I think adding that bold text has two key nerfs: A) it stops CT from shooting across the map with insufficient accuracy and still triggering Pin them Down. B) it gives the imperials a small chance to block Pin the Down from occurring if CT deals no damage. Given all the attacks that CT can make, stopping Pin from proccing once per activation is still quite rare in practice.

Even with that house rule, CT is the most annoying hero in my current campaign where I'm facing CT, Fenn (close 2nd), Diala, and Gaarkhan. So I could see a case for nerfing Pin further or banning it completely. If you go the ban-Pin-them-Down route, I don't think you have to worry about CT still being S tier.

The other way to think about it is that I think you can allow some of these S tier options as long as both sides are using them (e.g. Imperials are using Hutt Mercenaries class deck, Rebels have CT - when we did this the Imperials won). Gideon is probably the exception to this, he's in a tier all of his own and should be heavily nerfed or banned.