Support staff are incredible – especially secretaries who often have your back when times are tough. There’s nothing I’d really differentiate here for a professional setting: I’d speak to the trainee (alone) and tell them I didn’t approve. I’d be calm but firm, and I’d probably check in with the support staff. If the trainee is difficult I just wouldn’t interact with him/her. If it continued or it escalated I’d probably speak to my supervisor or someone at the firm.