If you don't want to keep your domain name, then you can easily indicate in our system that it shouldn't be renewed.
This way, the domain name will be deleted on its expiry date. Until that time, you will still be able to change your mind. With many extensions, also after the expiry date it might be possible to reactivate the domain name, but often then the registry does apply an extra charge.
If you want to also already technically disable your domain name, then you can do so by going to the nameserver settings of the domain name and under "bNamed" nameservers, remove all records.