It is not possible to remove someone from a Freehand project they have previously viewed. Even after removing them from the team, they will still have access to the Freehand.
To remove users from your team:
Sign in to your account and navigate to the People page.
Hover over the member and click View Profile.
Beneath the member's name and profile icon, click the ••• (more) menu.
Click Remove from Team.
To confirm, click Yes, Remove from Team.
If you navigate back to the People tab, you should notice that the member is gone.
As noted above, even after removing someone from your InVision V6 team, that person can still access any freehand they had previously viewed. This could cause some confusion if you migrate to InVision V7, because any freehand collaborator who was previously removed from your InVision V6 team will automatically be added to your InVision V7 team account. Should you run into this situation, you can quickly remove the user from you InVision V7 team.