I dont see how bringing up shotguns helps anybody.
I think any human being reading this thread can understand what an unpleasant experience it is to be disciplined by a supervisor, and to suggest that operators are testing supervisors by continuing to damage company property is a little far-fetched. Perhaps the ongoing damage is the result of an unacceptable level of operator error or perhaps the flow of lift trucks through the plant can be optimised to minimise contact with obstacles. Perhaps the high level of error is the result of undue pressure to move material as quickly as possible. Do you have drivers with a drug problem, a disability or an unsuitable personality (impatience) are they operating these trucks in cramped quarters, or do you simply not have enough operators and trucks to comfortably move the material required without hustling too fast?
If you are operating in a union environment your best bet is to look for a systemic solution rather than a people solution. Getting draconian with discipline will only result in reduced morale and more combative behaviour from your workers and your union rep. So concentrate on finding ways to smooth the flow of traffic through the plant and weed-out and reassign the operators with high incidences of damage, find ways to relieve pressure on operator output and you will almost certainly see a reduction of damage. Also realise you will likely never reduce damage to zero with twenty trucks and if you expect and strive for absolute perfection, you will only add unnecessary stress and animosity for everyone. Good luck