Can I restrict a team member's access to a specific campaign?

Yes, it's possible to restrict a team member's access on a per-campaign basis. To do this, head over to your Team Member page and either invite a new team member or click on an existing user. 

From there, you'll be able to restrict the access that your team member has to one specific campaign.

Note: team members who have been added as another admin user will have access to all campaigns. All other added team members can have access restricted to one campaign.