Identify the members of the team that encourage bad behaviour. Set clear expectations in terms of manners and to keep things professional (there’s no space here for personal conflicts that go beyond work disagreements). Identify a few team members who could be role models and let them – with my support – lead productive meetings. It also sounds to me like there are things that are or have been unsaid, and that may have accumulated over time, so there may some things that will require more effort to sort out. If after some time people don’t live up to the new expectations, unfortunately this will have to carry consequences (in terms of either moving people to other roles/teams/outside). The team should under the context and the role they play together for the company and that their collective behaviour is negatively impacting their productivity and reputation in the firm. I may need to get help from outside if there are serious cases of misbehaviour that require a lot of attention and energy (up to the point where unfortunately someone has had enough chance to improve his/her behaviour and we have to talk about next steps).