If you are an Owner or an Admin, you can remove people from certain projects or from the team overall.
Deleting a person from a project
To delete a person form a project, open the Team & Resources tab from the left sidebar. Select a corresponding person and click on Remove on the right to the required project. 
Alternatively, open the People tab of a particular project and click on Remove on the right to the corresponding person.
This person will be deleted from the project and tasks from this project s/he is assigned to.
Deleting a person from a team
To delete a person from a team, open the Team & Resources tab from the left sidebar. Then click on the three-dots button on the right of that person and select Delete resource.
This person will be deleted from all the projects and tasks s/he is assigned to. The estimation will be deleted as well, while logged time will be kept in tasks and included in a time log report.
A person removed from the project or the team will get an email notification about that.