I just did a bad, bad thing. I got rid of our First Generation Nautilus four-way neck machine (and abdominal crunch machine), knowing full well that I was going to have a mutiny on my hands from my older members who have been using that machine since the early 70s. My staff and I have been handing out information about the anatomy of the Atlas and Axis, other neck exercises and taking people by the hand to ensure them that this decision was made in their best interest (especially since we are not catering our services to a football program, just the recreational athlete and average joe). So it seems that this isn't enough. I was hoping you would be able to lend a helping hand (as you have done in the past) to help us thwart this onslaught of angry OLD men.
Be direct. A wise man once said, "You can't get peoples' attention anymore by tapping them on the shoulder. You gotta hit 'em over the head with a sledge hammer!" Obviously, this is a little extreme, but what you could try doing is taking the member(s) to a full length mirror. Then explain and show them the concepts of optimal spinal alignment, specifically at the cervical spine. Since a chronic four-way neck user is bound to have a forward head to some degree, you might show them the difference between OPTIMAL cervical posture and THEIR posture. Ask them if they've had stiffness, neck pain or spasms and headaches (which they most likely will have had at some point, if exposed to this machine for any length of time), and inform them that this machine very well may be the root of the problem.
Another technique you might use is to have them do a crunch on a ball and/or do a set of push ups. Absolutely do not let the cervical spine deviate from neutral! The member will ultimately display a significant muscular imbalance by way of extreme cervical protraction (due to tight and short sternocleidomastoids, scalenes and upper traps, along with weak deep cervical flexors). You might then explain to them again that this inability to maintain optimal cervical posture is/was being encouraged by the four-way neck machine and may have painful repercussions sooner or later.
You might then inform them that if they worked on or received guidance (personal training) on proper cervical flexibility/alignment, this alone will increase neck strength, not to mention comfort and ROM. I have found in similar situations as this one that by simply showing someone his or her own less than perfect posture or motion and then demonstrating what the posture should be or what the movement should look like, it's enough to at least get members/clients curious for more info.