How can I keep subscribers from changing their address (or going on vacation or switching to the Digest)?
Each email contains a link allowing users to Unsubscribe, per the CAN-SPAM Act. This link can be edited to allow users to manage their subscription, as well.
Subscribers can change their email address, put their account on vacation hold, or switch to/from the Digest. If preferred, you can restrict subscribers from making any or all of these changes to their account.
For more details on how to change these settings, please see this post.