Once you no longer need it, you can always delete a Mautic instance along with all related migrations, services etc.
Deleting an instance also releases all delegated resources, which become available to be used by other instances.
To delete an instance click on the gearbox icon next to it and then click Delete.
delete in the field to confirm the deletion and click Delete.
The deletion process is automated, but not instant and can take up to a few minutes to complete.