Only the developers have the possibility to issue a full refund to the payment method used to acquire the software. You have to contact them to ask for a refund and if you meet their requirements, then they should do it without any hesitation.
Contact info: https://www.slimwareutilities.com/contact_us.php
Secondly, try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
Revo will delete and uninstall everything about the software that is found on your PC.
