Yes, you can opt out of a JC, although you can't do it repeatedly without raising some suspicions, or at least some questions. You are sort of expected to take your "turn," however I've seen guys blow off several such "privileges" in a row because of work schedules, going on vacation, etc. so, if an elder simply did not want to be on a JC, it is possible to forestall for months at a time, then "cherry pick" an easy one and start dodging assignments all over again.
Having said that, I served on one several months before I launched my fade. Because of various circumstances it would have been awkward to refuse this particular one, and anyway it seemed pretty cut and dried at the time. A sister had kicked out her dub husband and now, some months later, was sharing her bed with a fellow she worked with. This was obvious to several of her relatives, including her MS son, who ratted her out. We made several phone calls to set up an appointment to discuss this with her, but she was apparently screening her calls and never picked up, nor did she return any calls. Another elder and I went by her house early one evening, thinking she might at least answer the door. When we pulled up, cars were parked in the driveway and the front door was open. As we walked up the sidewalk to the metal gate, we heard the TV volume go way down and heard voices within. Then it got very quiet and the dark screen door opened slightly and out came this huge ugly dog who proceeded to bound over to the gate and snarl at us.
We looked at each other and turned around and left. We continued to leave messages. One day a child answered and said, yes, her mom was home. After several minutes she picked up the phone and we talked. She readily admitted she was living with the guy and said she didn't care what we did about it. We said we'd have to schedule a meeting and would let her know when it was; her reply was, don't bother, I won't be there. She also shared some sad stories about her JW ex, including his abusive nature and some other quirks that would have gotten him in a lot of trouble except that he had moved to parts unknown and was not associating with any KH that we knew of.
We met, df'd her on the "evidence" which consisted of her admission to the two of us on the phone (her admission to her son, the MS, was a bonus, I guess). We called her after the brief meeting and she took the call. We told her what we decided. She said okay, and hung up. We felt icky. I still do. I often think of calling her and apologizing and telling her I'm glad she found some love in her life for a change and that I hope she's happy. I don't do this, only because I know her MS son and other JW relatives still visit her (there are several grandchildren) and she'd tell them and I'd be outed. However, being outed is less and less a concern to me and my family with each passing month, so the time will come when I will. It's just one of the things you have to live down when you devote your life to a cult. If there is a hell, I'm cooked, I suppose, but perhaps if I live long enough I can post enough mitigating circumstances to my account that I'll catch a break.