Your users can
- say to Discourse “don’t send me digest emails” from their profile
/preferences/emails. Just disable the entry “When I do not visit here, send me an email summary of popular topics and replies” - unsubscribe the digest email
You can
- disable digest email globally, see the site setting
disable digest emails - disable digest email for users not seen on the site for more than (n) days (see the site setting
suppress digest email after days)