Provided that your camp is labeling emails with the correct "type", parents should only wish to opt out of "marketing" type emails. An admin user cannot edit a parent's marketing email preferences on their behalf. Parents can opt out of receiving marketing emails from your camp by clicking the "unsubscribe" link at the bottom of any marketing email they receive, or by updating their Email Preferences from the "My Account" page of their parent dashboard.
If your camp is mis-labeling promotional emails as "informational", the recipient will not be able to opt-out/unsubscribe, in which case they may mark your messages as spam. This will prevent the recipient from receiving ANY emails from you moving forward - including potentially important emails about their camper's enrollment.