The subject line of the confirmation email that is sent to the subscriber can be changed when you add or edit the block (it's the field labelled "Confirmation Email SUBJECT").

The subject lines for the 2 notification emails that are sent to the site admin are hard-coded. The first one (sent to the admin when someone fills out the form) is line 119 of packages/email_list_signup/blocks/email_list_signup/controller.php. The second one (sent to the admin when the subscriber clicks the confirmation link in their email) is line 41 of packages/email_list_signup/controllers/confirm_signup.php.

-Jordan