Get Your Students Hired
Try and find out why your non-serious student is there in the first place. Maybe they feel out of place, intimidated, etc. and are using non-serious behavior to mask it. Not that I am making excuses since they are adults they should be responsible for their own behavior and respect those around them: classmates and instructor alike since you are all there to do a job: learn and teach respectively. Perhaps discuss the issue "hypothetically" as a group if it is possible so students can try and see both sides of the story while looking at it from the outside and not their experience.