CBSubs Mailer must be able to email also to users that have been blocked, e.g. by a CBSubs plan.
If that is unwanted, you can just edit the settings of the mailer in Group tab, Buyer usergroup conditions,
Applicable for View Access Level and change it from Public to Registered if your View Access levels correspond to that normal situation, otherwise you can select joomla user groups specifically below that setting.
EDIT: That will not fix the blocked user issue! We need a separate setting for this! Added a FR to implement this:
forge.joomlapolis.com/issues/7954