You can remove anyone from a Conversation's membership bar by clicking in the membership bar, then clicking Manage. However, you can't remove the name and avatar from posts someone made before leaving the company, because that person might have been assigned flags or assigned flags to others. Leaving the person's posts in Conversations allows other Conversation participants to decide how to deal with those flags; for example, reassigning or simply clearing them. (Note that the person's account is disabled, so the avatar should be in permanent offline status.) In addition, although someone might have left the company, his or her posts might still supply valuable information, and they also provide an accurate historical record of what transpired within the Conversation(s). Stripping the person's name and avatar from those posts would essentially leave those posts unattributed, likely causing a great deal of confusion for anyone reading the Conversation end to end.