Has anybody ever done ballistic bench press on a smith machine in a commercial gym? If so did you have a problem with any of the trainers telling you that you couldn’t do that?
Not I, but I could certainly see their point.
The goal of any successful gym chain isn’t to improve fitness; it’s to minimize liability. They’d just as soon have all of you using pink dumbbells, just in case you accidentally drop the heavier chrome models on your toes and decide to sue.
Think how you must look to the staff; an exercise of highly questionable benefit and shockingly unsafe variety performed by a person who skipped the Intro To Weightlifting class, and is therefore unsuited to anything beyond the realm of Body For Life.
Of course they’d stand in your way.
The solution is to become so totally huge that you either A) MUST know what you’re doing, or B) intimidate them from stepping within spitballing distance of your workout.
DI
Tell them they can’t stop you, because you’re more explosive than they are. 
I have not had a problem with the staff at my gym when I do them. I usually work out early in the morning, so there are not a lot of people there and the staff is not fully awake. I have yet to try them on Bench Press Night :-), I mean Monday nights, so there has not been an issue.
Oh, how I hate Monday nights. Not only is it Bench Press Nite, it also seems to be Chiropractor Curl Nite over in the squat racks. Fortunately, you won’t see 80% of the people back in the gym until the following Monday.
Who cares? The whole point of doing them in the smith machine is you can set the stops and thus the movement is completely safe, right?
Even if they gave you shit I’m sure you could explain to them what you’re doing and they’ll go away. If not, get angry and they’ll go away. 
I do it at my gym and I am a trainer I do get looks from the members though. But thats a good thing because they wind up asking me questions I would eventually pick the up as a client down the road.
The trainer’s in my gym are all pretty good they really know what they are doing for the most part. Most of them about 99% have a college degree in Exercise Phis or Science, something like that And we are either ACSM or CSCS or NSAM certfied. There’s really no rookie trainer or unknowledgable trainer’s.
Then agian I do work for a corporate fitness facility and they do pay top dollars for quality service.
In Health,
Silas C.