When I observed jury selection I saw several people back down from stances that were clearly attempts to get kicked off the jury.
It's not like the lawyers ask one question then dismiss you.
I did see one guy who claimed he believed "all courts are invalid" hold up to some pretty intense questioning before getting dismissed. But by the grin on his face I could tell he relished winning that one.
The reasons that seemed to get a quick dismissal were people who had been victims of crime in the past, people who had family members who were police officers, lawyers, judges. People who held not uncommon beliefs that would bias them (e.g. "I believe that people of color can't get a fair trial"), but not always if the juror said they could make a decision without that bias clouding their judgement.
The ones that picked extreme beliefs just got more questioning until they either admitted their belief wouldn't cloud their judge or that their belief wasn't so extreme as to bias them.
I mean, the judge and lawyers question people for a living, so they know exactly how to drill down and force an answer out.
No one, not the judge, the prosecutors or the defense want jury selection to take a long time and there are only so many people in the pool to choose from.
That said, on a different trial, a serial killer, it took almost 6 weeks to panel a jury. I was there for a week before being dismissed because I’m morally against the death penalty. Even then the judge interviewed me for 30 minutes on this point, including about my religious upbringing as a child.
It's not illegal in any way. It won't get you thrown in jail for contempt. It is probably protected by the first ammendment to say so. It will get you excluded from the jury.
People do it all the time, in fact.
The exact standard questions asked of jurors in voir dire vary from jurisdiction to jurisdiction, but usually include some variation of if there any religious, philosophical, or ethical reasons you could not convict (or not acquit) generally.
I mean, imagine if you had philosophical, ethical, or reliigous reasons you couldn't convict under any circumstances and you wound up on the jury anyway! If you didn't tell the court this when asked and wound up on the jury cause you lied about it... you probably still wouldn't actually be considered in contempt, but it'd be bad.
If you say in voir dire that you wouldn't vote to convict even if the person is guilty, you're not going to get put on a jury.