Great question! I have had to deal with this issue and it is not fun. I have dealt with it two ways and both have worked.

Option 1 - I had a discussion with the difficult person about my hesitation in placing them in a group. I told them about their bad habits and how their negativity and other issues were dominating and killing the group. After the discussion I placed the person with a very strong small group leader that held them accountable and the person became a healthy small group member.

Option 2 - I place another difficult person with a strong leader that was very skilled at leading discussions. When the difficult person tried to take over and dominate discussion the leader corrected the behavior in the moment over and over again and re-directed the discussion. After many challenging meetings the leader had a similar discussion as I had (with a different person) in option 1. Unfortunately this person was eventually asked to leave the group by the leader due to a need for professional counseling since the needs way outgrew the groups span of care.

I hope that helps