The incorrect use of a machine can endanger everyone in the building. As a result of this shared danger, we all share the responsibility to police each other. We each have a responsibility to step up and question anyone using a machine in a way we feel is unsafe. This includes questioning the operator about the materials they are cutting.
If you are operating a machine and someone questions you about the safety of your operation, you should not be offended even if their approach to you is poor.
If someone asks you to stop what you are doing, for any reason, then you must pause your work. Talk about their concern. If you cannot resolve the concern between you, then seek out a staff member.
Bottom line: No matter what you are doing on a machine, no matter how important your job is, if someone asks you to stop for a concern about safety, you are obligated to stop immediately. If you do not then you may lose your access to the makerspace even if you are later proven correct.