If the artist declined the promo invite that was sent to them, it's still possible for them to change their mind and accept email promos from your label.
To do so, they need to get access to their subscriptions list. They can do this by following the link from any of their previous promotion emails, including the original invite.
Alternatively, they can access their promos and subscription options from the "Promo" link at the bottom of the Label Engine homepage - https://label-engine.com/promo_access.php. From this page they will need to enter in the email address that you send promos to, upon doing so they will receive an email containing a link to view their promos.
Once they are in the Label Engine promo system, they should click the 'Subscriptions' link at the top of the page and then find your label. Under the label name and logo they will see a link that says 'Please opt-in to receive promo emails' - clicking that will show them a pop-up that allows them to select their delivery options, and then they can click save to confirm the opt-in.