When a user is removed from a project, all issues where this user is an assignee, reporter, or watcher stay in the project. The user's name in all issue fields changes to their email address.
To find all the issues where the user is involved, apply a filter by Assignee, Reporter, or Watcher and enter their email address. Then use the "Multi edit" tool to reassign or delete the issues.
Remember to reassign the issues with the "Closed" status.