To remove a user from a project, click the member avatars on the right panel. On the members modal, click the "..." options menu next to the username, then select "Remove from the project."

When adding a user to a project, you may notice a list of suggested users. These suggestions are members of other projects that you also belong to or own. To remove a user from this list of invite suggestions, they will need to be removed from each of these projects, including archived ones.