Yes, Nimoy talks about Shatner quitting in their conversation on the Mind Meld DVD. Shatner says a big reason he quit was because his young daughters would recoil at the smell and not want to hug him. Shatner quit cold turkey, IIRC. Nimoy remembered him at one point going off to a corner of the set and screaming at the top of his lungs, "I WANT A CIGARETTE!!!" But he didn't crack. He quit out of sheer willpower.If I recall correctly, Shatner smoked but quit during the original run.
Shatner says later on (I think while they were discussing Nimoy and Nerine Shatner's battles with alcohol), "I don't have an addictive personality." Nimoy quietly says, "I do," which might explain why he had so much more trouble quitting.